diff --git a/changelog.d/14565.misc b/changelog.d/14565.misc
new file mode 100644
index 000000000000..19a62b036c8f
--- /dev/null
+++ b/changelog.d/14565.misc
@@ -0,0 +1 @@
+In application service transactions that include the experimental `org.matrix.msc3202.device_one_time_key_counts` key, include a duplicate key of `org.matrix.msc3202.device_one_time_keys_count` to match the name proposed by [MSC3202](https://github.com/matrix-org/matrix-spec-proposals/blob/travis/msc/otk-dl-appservice/proposals/3202-encrypted-appservices.md).
diff --git a/synapse/appservice/__init__.py b/synapse/appservice/__init__.py
index 500bdde3a986..bf4e6c629b38 100644
--- a/synapse/appservice/__init__.py
+++ b/synapse/appservice/__init__.py
@@ -32,9 +32,9 @@
 
 logger = logging.getLogger(__name__)
 
-# Type for the `device_one_time_key_counts` field in an appservice transaction
+# Type for the `device_one_time_keys_count` field in an appservice transaction
 #   user ID -> {device ID -> {algorithm -> count}}
-TransactionOneTimeKeyCounts = Dict[str, Dict[str, Dict[str, int]]]
+TransactionOneTimeKeysCount = Dict[str, Dict[str, Dict[str, int]]]
 
 # Type for the `device_unused_fallback_key_types` field in an appservice transaction
 #   user ID -> {device ID -> [algorithm]}
@@ -376,7 +376,7 @@ def __init__(
         events: List[EventBase],
         ephemeral: List[JsonDict],
         to_device_messages: List[JsonDict],
-        one_time_key_counts: TransactionOneTimeKeyCounts,
+        one_time_keys_count: TransactionOneTimeKeysCount,
         unused_fallback_keys: TransactionUnusedFallbackKeys,
         device_list_summary: DeviceListUpdates,
     ):
@@ -385,7 +385,7 @@ def __init__(
         self.events = events
         self.ephemeral = ephemeral
         self.to_device_messages = to_device_messages
-        self.one_time_key_counts = one_time_key_counts
+        self.one_time_keys_count = one_time_keys_count
         self.unused_fallback_keys = unused_fallback_keys
         self.device_list_summary = device_list_summary
 
@@ -402,7 +402,7 @@ async def send(self, as_api: "ApplicationServiceApi") -> bool:
             events=self.events,
             ephemeral=self.ephemeral,
             to_device_messages=self.to_device_messages,
-            one_time_key_counts=self.one_time_key_counts,
+            one_time_keys_count=self.one_time_keys_count,
             unused_fallback_keys=self.unused_fallback_keys,
             device_list_summary=self.device_list_summary,
             txn_id=self.id,
diff --git a/synapse/appservice/api.py b/synapse/appservice/api.py
index 60774b240d9f..edafd433cda3 100644
--- a/synapse/appservice/api.py
+++ b/synapse/appservice/api.py
@@ -23,7 +23,7 @@
 from synapse.api.errors import CodeMessageException
 from synapse.appservice import (
     ApplicationService,
-    TransactionOneTimeKeyCounts,
+    TransactionOneTimeKeysCount,
     TransactionUnusedFallbackKeys,
 )
 from synapse.events import EventBase
@@ -262,7 +262,7 @@ async def push_bulk(
         events: List[EventBase],
         ephemeral: List[JsonDict],
         to_device_messages: List[JsonDict],
-        one_time_key_counts: TransactionOneTimeKeyCounts,
+        one_time_keys_count: TransactionOneTimeKeysCount,
         unused_fallback_keys: TransactionUnusedFallbackKeys,
         device_list_summary: DeviceListUpdates,
         txn_id: Optional[int] = None,
@@ -310,10 +310,13 @@ async def push_bulk(
 
         # TODO: Update to stable prefixes once MSC3202 completes FCP merge
         if service.msc3202_transaction_extensions:
-            if one_time_key_counts:
+            if one_time_keys_count:
                 body[
                     "org.matrix.msc3202.device_one_time_key_counts"
-                ] = one_time_key_counts
+                ] = one_time_keys_count
+                body[
+                    "org.matrix.msc3202.device_one_time_keys_count"
+                ] = one_time_keys_count
             if unused_fallback_keys:
                 body[
                     "org.matrix.msc3202.device_unused_fallback_key_types"
diff --git a/synapse/appservice/scheduler.py b/synapse/appservice/scheduler.py
index 430ffbcd1fc8..7b562795a3f2 100644
--- a/synapse/appservice/scheduler.py
+++ b/synapse/appservice/scheduler.py
@@ -64,7 +64,7 @@
 from synapse.appservice import (
     ApplicationService,
     ApplicationServiceState,
-    TransactionOneTimeKeyCounts,
+    TransactionOneTimeKeysCount,
     TransactionUnusedFallbackKeys,
 )
 from synapse.appservice.api import ApplicationServiceApi
@@ -258,7 +258,7 @@ async def _send_request(self, service: ApplicationService) -> None:
                 ):
                     return
 
-                one_time_key_counts: Optional[TransactionOneTimeKeyCounts] = None
+                one_time_keys_count: Optional[TransactionOneTimeKeysCount] = None
                 unused_fallback_keys: Optional[TransactionUnusedFallbackKeys] = None
 
                 if (
@@ -269,7 +269,7 @@ async def _send_request(self, service: ApplicationService) -> None:
                     # for the users which are mentioned in this transaction,
                     # as well as the appservice's sender.
                     (
-                        one_time_key_counts,
+                        one_time_keys_count,
                         unused_fallback_keys,
                     ) = await self._compute_msc3202_otk_counts_and_fallback_keys(
                         service, events, ephemeral, to_device_messages_to_send
@@ -281,7 +281,7 @@ async def _send_request(self, service: ApplicationService) -> None:
                         events,
                         ephemeral,
                         to_device_messages_to_send,
-                        one_time_key_counts,
+                        one_time_keys_count,
                         unused_fallback_keys,
                         device_list_summary,
                     )
@@ -296,7 +296,7 @@ async def _compute_msc3202_otk_counts_and_fallback_keys(
         events: Iterable[EventBase],
         ephemerals: Iterable[JsonDict],
         to_device_messages: Iterable[JsonDict],
-    ) -> Tuple[TransactionOneTimeKeyCounts, TransactionUnusedFallbackKeys]:
+    ) -> Tuple[TransactionOneTimeKeysCount, TransactionUnusedFallbackKeys]:
         """
         Given a list of the events, ephemeral messages and to-device messages,
         - first computes a list of application services users that may have
@@ -367,7 +367,7 @@ async def send(
         events: List[EventBase],
         ephemeral: Optional[List[JsonDict]] = None,
         to_device_messages: Optional[List[JsonDict]] = None,
-        one_time_key_counts: Optional[TransactionOneTimeKeyCounts] = None,
+        one_time_keys_count: Optional[TransactionOneTimeKeysCount] = None,
         unused_fallback_keys: Optional[TransactionUnusedFallbackKeys] = None,
         device_list_summary: Optional[DeviceListUpdates] = None,
     ) -> None:
@@ -380,7 +380,7 @@ async def send(
             events: The persistent events to include in the transaction.
             ephemeral: The ephemeral events to include in the transaction.
             to_device_messages: The to-device messages to include in the transaction.
-            one_time_key_counts: Counts of remaining one-time keys for relevant
+            one_time_keys_count: Counts of remaining one-time keys for relevant
                 appservice devices in the transaction.
             unused_fallback_keys: Lists of unused fallback keys for relevant
                 appservice devices in the transaction.
@@ -397,7 +397,7 @@ async def send(
                 events=events,
                 ephemeral=ephemeral or [],
                 to_device_messages=to_device_messages or [],
-                one_time_key_counts=one_time_key_counts or {},
+                one_time_keys_count=one_time_keys_count or {},
                 unused_fallback_keys=unused_fallback_keys or {},
                 device_list_summary=device_list_summary or DeviceListUpdates(),
             )
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 259456b55dab..c8858b22ddf6 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1426,14 +1426,14 @@ async def generate_sync_result(
 
         logger.debug("Fetching OTK data")
         device_id = sync_config.device_id
-        one_time_key_counts: JsonDict = {}
+        one_time_keys_count: JsonDict = {}
         unused_fallback_key_types: List[str] = []
         if device_id:
             # TODO: We should have a way to let clients differentiate between the states of:
             #   * no change in OTK count since the provided since token
             #   * the server has zero OTKs left for this device
             #  Spec issue: https://github.com/matrix-org/matrix-doc/issues/3298
-            one_time_key_counts = await self.store.count_e2e_one_time_keys(
+            one_time_keys_count = await self.store.count_e2e_one_time_keys(
                 user_id, device_id
             )
             unused_fallback_key_types = (
@@ -1463,7 +1463,7 @@ async def generate_sync_result(
             archived=sync_result_builder.archived,
             to_device=sync_result_builder.to_device,
             device_lists=device_lists,
-            device_one_time_keys_count=one_time_key_counts,
+            device_one_time_keys_count=one_time_keys_count,
             device_unused_fallback_key_types=unused_fallback_key_types,
             next_batch=sync_result_builder.now_token,
         )
diff --git a/synapse/storage/databases/main/appservice.py b/synapse/storage/databases/main/appservice.py
index 25da0c56c5d1..c2c8018ee255 100644
--- a/synapse/storage/databases/main/appservice.py
+++ b/synapse/storage/databases/main/appservice.py
@@ -20,7 +20,7 @@
     ApplicationService,
     ApplicationServiceState,
     AppServiceTransaction,
-    TransactionOneTimeKeyCounts,
+    TransactionOneTimeKeysCount,
     TransactionUnusedFallbackKeys,
 )
 from synapse.config.appservice import load_appservices
@@ -260,7 +260,7 @@ async def create_appservice_txn(
         events: List[EventBase],
         ephemeral: List[JsonDict],
         to_device_messages: List[JsonDict],
-        one_time_key_counts: TransactionOneTimeKeyCounts,
+        one_time_keys_count: TransactionOneTimeKeysCount,
         unused_fallback_keys: TransactionUnusedFallbackKeys,
         device_list_summary: DeviceListUpdates,
     ) -> AppServiceTransaction:
@@ -273,7 +273,7 @@ async def create_appservice_txn(
             events: A list of persistent events to put in the transaction.
             ephemeral: A list of ephemeral events to put in the transaction.
             to_device_messages: A list of to-device messages to put in the transaction.
-            one_time_key_counts: Counts of remaining one-time keys for relevant
+            one_time_keys_count: Counts of remaining one-time keys for relevant
                 appservice devices in the transaction.
             unused_fallback_keys: Lists of unused fallback keys for relevant
                 appservice devices in the transaction.
@@ -299,7 +299,7 @@ def _create_appservice_txn(txn: LoggingTransaction) -> AppServiceTransaction:
                 events=events,
                 ephemeral=ephemeral,
                 to_device_messages=to_device_messages,
-                one_time_key_counts=one_time_key_counts,
+                one_time_keys_count=one_time_keys_count,
                 unused_fallback_keys=unused_fallback_keys,
                 device_list_summary=device_list_summary,
             )
@@ -379,7 +379,7 @@ def _get_oldest_unsent_txn(
             events=events,
             ephemeral=[],
             to_device_messages=[],
-            one_time_key_counts={},
+            one_time_keys_count={},
             unused_fallback_keys={},
             device_list_summary=DeviceListUpdates(),
         )
diff --git a/synapse/storage/databases/main/end_to_end_keys.py b/synapse/storage/databases/main/end_to_end_keys.py
index cf33e73e2b4f..643c47d608da 100644
--- a/synapse/storage/databases/main/end_to_end_keys.py
+++ b/synapse/storage/databases/main/end_to_end_keys.py
@@ -33,7 +33,7 @@
 
 from synapse.api.constants import DeviceKeyAlgorithms
 from synapse.appservice import (
-    TransactionOneTimeKeyCounts,
+    TransactionOneTimeKeysCount,
     TransactionUnusedFallbackKeys,
 )
 from synapse.logging.opentracing import log_kv, set_tag, trace
@@ -514,7 +514,7 @@ def _count_e2e_one_time_keys(txn: LoggingTransaction) -> Dict[str, int]:
 
     async def count_bulk_e2e_one_time_keys_for_as(
         self, user_ids: Collection[str]
-    ) -> TransactionOneTimeKeyCounts:
+    ) -> TransactionOneTimeKeysCount:
         """
         Counts, in bulk, the one-time keys for all the users specified.
         Intended to be used by application services for populating OTK counts in
@@ -528,7 +528,7 @@ async def count_bulk_e2e_one_time_keys_for_as(
 
         def _count_bulk_e2e_one_time_keys_txn(
             txn: LoggingTransaction,
-        ) -> TransactionOneTimeKeyCounts:
+        ) -> TransactionOneTimeKeysCount:
             user_in_where_clause, user_parameters = make_in_list_sql_clause(
                 self.database_engine, "user_id", user_ids
             )
@@ -541,7 +541,7 @@ def _count_bulk_e2e_one_time_keys_txn(
             """
             txn.execute(sql, user_parameters)
 
-            result: TransactionOneTimeKeyCounts = {}
+            result: TransactionOneTimeKeysCount = {}
 
             for user_id, device_id, algorithm, count in txn:
                 # We deliberately construct empty dictionaries for
diff --git a/tests/appservice/test_scheduler.py b/tests/appservice/test_scheduler.py
index 0b22afdc7598..0a1ae83a2bbf 100644
--- a/tests/appservice/test_scheduler.py
+++ b/tests/appservice/test_scheduler.py
@@ -69,7 +69,7 @@ def test_single_service_up_txn_sent(self):
             events=events,
             ephemeral=[],
             to_device_messages=[],  # txn made and saved
-            one_time_key_counts={},
+            one_time_keys_count={},
             unused_fallback_keys={},
             device_list_summary=DeviceListUpdates(),
         )
@@ -96,7 +96,7 @@ def test_single_service_down(self):
             events=events,
             ephemeral=[],
             to_device_messages=[],  # txn made and saved
-            one_time_key_counts={},
+            one_time_keys_count={},
             unused_fallback_keys={},
             device_list_summary=DeviceListUpdates(),
         )
@@ -125,7 +125,7 @@ def test_single_service_up_txn_not_sent(self):
             events=events,
             ephemeral=[],
             to_device_messages=[],
-            one_time_key_counts={},
+            one_time_keys_count={},
             unused_fallback_keys={},
             device_list_summary=DeviceListUpdates(),
         )
diff --git a/tests/handlers/test_appservice.py b/tests/handlers/test_appservice.py
index 144e49d0fd9c..9ed26d87a753 100644
--- a/tests/handlers/test_appservice.py
+++ b/tests/handlers/test_appservice.py
@@ -25,7 +25,7 @@
 from synapse.api.constants import EduTypes, EventTypes
 from synapse.appservice import (
     ApplicationService,
-    TransactionOneTimeKeyCounts,
+    TransactionOneTimeKeysCount,
     TransactionUnusedFallbackKeys,
 )
 from synapse.handlers.appservice import ApplicationServicesHandler
@@ -1123,7 +1123,7 @@ def test_application_services_receive_otk_counts_and_fallback_key_usages_with_pd
         # Capture what was sent as an AS transaction.
         self.send_mock.assert_called()
         last_args, _last_kwargs = self.send_mock.call_args
-        otks: Optional[TransactionOneTimeKeyCounts] = last_args[self.ARG_OTK_COUNTS]
+        otks: Optional[TransactionOneTimeKeysCount] = last_args[self.ARG_OTK_COUNTS]
         unused_fallbacks: Optional[TransactionUnusedFallbackKeys] = last_args[
             self.ARG_FALLBACK_KEYS
         ]