diff --git a/packages/merge-styles/etc/merge-styles.api.md b/packages/merge-styles/etc/merge-styles.api.md
index ae90c9064ab7b..d9f2a075f155f 100644
--- a/packages/merge-styles/etc/merge-styles.api.md
+++ b/packages/merge-styles/etc/merge-styles.api.md
@@ -65,7 +65,7 @@ export const GLOBAL_STYLESHEET_KEY = "__global__";
 //
 // @public
 export type IConcatenatedStyleSet<TStyleSet extends IStyleSetBase> = {
-    [P in keyof Omit_2<TStyleSet, 'subComponentStyles'>]: IStyle;
+    [P in keyof Omit<TStyleSet, 'subComponentStyles'>]: IStyle;
 } & {
     subComponentStyles?: {
         [P in keyof TStyleSet['subComponentStyles']]: IStyleFunction<any, any>;
@@ -114,7 +114,7 @@ export type InsertRuleCallback = ({ key, sheet, rule }: InsertRuleArgs) => void;
 
 // @public
 export type IProcessedStyleSet<TStyleSet extends IStyleSetBase> = {
-    [P in keyof Omit_2<TStyleSet, 'subComponentStyles'>]: string;
+    [P in keyof Omit<TStyleSet, 'subComponentStyles'>]: string;
 } & {
     subComponentStyles: {
         [P in keyof TStyleSet['subComponentStyles']]: __MapToFunctionType<TStyleSet['subComponentStyles'] extends infer J ? (P extends keyof J ? J[P] : never) : never>;
@@ -466,7 +466,7 @@ export type IStyleFunctionOrObject<TStylesProps, TStyleSet extends IStyleSetBase
 export type IStyleSet<TStyleSet extends IStyleSetBase = {
     [key: string]: any;
 }> = {
-    [P in keyof Omit_2<TStyleSet, 'subComponentStyles'>]: IStyle;
+    [P in keyof Omit<TStyleSet, 'subComponentStyles'>]: IStyle;
 } & {
     subComponentStyles?: {
         [P in keyof TStyleSet['subComponentStyles']]: IStyleFunctionOrObject<any, any>;
@@ -569,12 +569,16 @@ export { Omit_2 as Omit }
 export function setRTL(isRTL: boolean): void;
 
 // @public (undocumented)
-export type ShadowConfig = {
-    stylesheetKey: string;
+export interface ShadowConfig {
+    // (undocumented)
+    __isShadowConfig__: true;
+    // (undocumented)
     inShadow: boolean;
+    // (undocumented)
+    stylesheetKey: string;
+    // (undocumented)
     window?: Window;
-    __isShadowConfig__: true;
-};
+}
 
 // @public (undocumented)
 export class ShadowDomStylesheet extends Stylesheet {
@@ -635,7 +639,7 @@ export const SUPPORTS_MODIFYING_ADOPTED_STYLESHEETS: boolean;
 // Warnings were encountered during analysis:
 //
 // lib/IRawStyle.d.ts:24:9 - (ae-forgotten-export) The symbol "IStyle_2" needs to be exported by the entry point index.d.ts
-// lib/IStyleSet.d.ts:61:5 - (ae-forgotten-export) The symbol "__MapToFunctionType" needs to be exported by the entry point index.d.ts
+// lib/IStyleSet.d.ts:62:5 - (ae-forgotten-export) The symbol "__MapToFunctionType" needs to be exported by the entry point index.d.ts
 
 // (No @packageDocumentation comment for this package)
 
diff --git a/packages/merge-styles/src/IStyleOptions.ts b/packages/merge-styles/src/IStyleOptions.ts
index 75ba01e0115a7..33f0d088153b1 100644
--- a/packages/merge-styles/src/IStyleOptions.ts
+++ b/packages/merge-styles/src/IStyleOptions.ts
@@ -1,4 +1,4 @@
-import { ShadowConfig } from './shadowConfig';
+import type { ShadowConfig } from './shadowConfig';
 import type { Stylesheet } from './Stylesheet';
 
 export interface IStyleOptions {
diff --git a/packages/merge-styles/src/IStyleSet.ts b/packages/merge-styles/src/IStyleSet.ts
index 7122b5b58d901..857e99c9d04eb 100644
--- a/packages/merge-styles/src/IStyleSet.ts
+++ b/packages/merge-styles/src/IStyleSet.ts
@@ -12,8 +12,10 @@ export type Diff<T extends keyof any, U extends keyof any> = ({ [P in T]: P } &
 /**
  * @deprecated Use the version provided by TypeScript instead.
  */
+// eslint-disable-next-line deprecation/deprecation, @typescript-eslint/naming-convention
+type _Omit<U, K extends keyof U> = Pick<U, Diff<keyof U, K>>;
 // eslint-disable-next-line deprecation/deprecation
-export type Omit<U, K extends keyof U> = Pick<U, Diff<keyof U, K>>;
+export type { _Omit as Omit };
 
 /**
  * Helper function whose role is supposed to express that regardless if T is a style object or style function,
@@ -38,7 +40,6 @@ export interface IStyleSetBase {
  * property.
  */
 export type IStyleSet<TStyleSet extends IStyleSetBase = { [key: string]: any }> = {
-  // eslint-disable-next-line deprecation/deprecation
   [P in keyof Omit<TStyleSet, 'subComponentStyles'>]: IStyle;
 } & {
   subComponentStyles?: { [P in keyof TStyleSet['subComponentStyles']]: IStyleFunctionOrObject<any, any> };
@@ -48,7 +49,6 @@ export type IStyleSet<TStyleSet extends IStyleSetBase = { [key: string]: any }>
  * A concatenated style set differs from `IStyleSet` in that subComponentStyles will always be a style function.
  */
 export type IConcatenatedStyleSet<TStyleSet extends IStyleSetBase> = {
-  // eslint-disable-next-line deprecation/deprecation
   [P in keyof Omit<TStyleSet, 'subComponentStyles'>]: IStyle;
 } & {
   subComponentStyles?: { [P in keyof TStyleSet['subComponentStyles']]: IStyleFunction<any, any> };
@@ -59,7 +59,6 @@ export type IConcatenatedStyleSet<TStyleSet extends IStyleSetBase> = {
  * into a class name. Additionally, all subComponentStyles are style functions.
  */
 export type IProcessedStyleSet<TStyleSet extends IStyleSetBase> = {
-  // eslint-disable-next-line deprecation/deprecation
   [P in keyof Omit<TStyleSet, 'subComponentStyles'>]: string;
 } & {
   subComponentStyles: {
diff --git a/packages/merge-styles/src/concatStyleSetsWithProps.ts b/packages/merge-styles/src/concatStyleSetsWithProps.ts
index 2834b38a90988..446189f3c21af 100644
--- a/packages/merge-styles/src/concatStyleSetsWithProps.ts
+++ b/packages/merge-styles/src/concatStyleSetsWithProps.ts
@@ -12,21 +12,21 @@ export function concatStyleSetsWithProps<TStyleProps, TStyleSet extends IStyleSe
   styleProps: TStyleProps,
   ...allStyles: (IStyleFunctionOrObject<TStyleProps, TStyleSet> | undefined)[]
 ): DeepPartialV2<TStyleSet> {
-  const result: DeepPartialV2<TStyleSet>[] = [];
+  const result: Array<DeepPartialV2<TStyleSet>> = [];
   for (const styles of allStyles) {
     if (styles) {
       result.push(typeof styles === 'function' ? styles(styleProps) : styles);
     }
   }
   if (result.length === 1) {
-    return result[0] as DeepPartialV2<TStyleSet>;
+    return result[0];
   } else if (result.length) {
     // cliffkoh: I cannot figure out how to avoid the cast to any here.
     // It is something to do with the use of Omit in IStyleSet.
     // It might not be necessary once  Omit becomes part of lib.d.ts (when we remove our own Omit and rely on
     // the official version).
-    return concatStyleSets(...result) as any;
+    return concatStyleSets(...result) as DeepPartialV2<TStyleSet>;
   }
 
-  return {} as any;
+  return {} as DeepPartialV2<TStyleSet>;
 }
diff --git a/packages/merge-styles/src/mergeStyleSets.ts b/packages/merge-styles/src/mergeStyleSets.ts
index 485a49f636537..f2fefbafb5926 100644
--- a/packages/merge-styles/src/mergeStyleSets.ts
+++ b/packages/merge-styles/src/mergeStyleSets.ts
@@ -179,12 +179,12 @@ export function mergeCssSets<TStyleSet>(
  * @param options - (optional) Options to use when creating rules.
  */
 export function mergeCssSets(styleSets: any[], options?: IStyleOptions): IProcessedStyleSet<any> {
-  const classNameSet: IProcessedStyleSet<any> = { subComponentStyles: {} };
+  const classNameSet = { subComponentStyles: {} } as IProcessedStyleSet<any>;
 
   let shadowConfig: ShadowConfig | undefined = undefined;
   let styleSet;
   if (isShadowConfig(styleSets[0])) {
-    shadowConfig = styleSets[0] as ShadowConfig;
+    shadowConfig = styleSets[0];
     styleSet = styleSets[1];
   } else {
     styleSet = styleSets[0];
@@ -207,7 +207,7 @@ export function mergeCssSets(styleSets: any[], options?: IStyleOptions): IProces
   for (const styleSetArea in concatenatedStyleSet) {
     if (concatenatedStyleSet.hasOwnProperty(styleSetArea)) {
       if (styleSetArea === 'subComponentStyles') {
-        classNameSet.subComponentStyles = (concatenatedStyleSet as IConcatenatedStyleSet<any>).subComponentStyles || {};
+        classNameSet.subComponentStyles = concatenatedStyleSet.subComponentStyles || {};
         continue;
       } else if (styleSetArea === '__shadowConfig__') {
         continue;
@@ -222,12 +222,10 @@ export function mergeCssSets(styleSets: any[], options?: IStyleOptions): IProces
 
         if (registration) {
           registrations.push(registration);
-          // FIXME: classNameSet invalid types - exposed in TS 4.5 - cast needed
-          (classNameSet as Record<string, any>)[styleSetArea] = classes.concat([registration.className]).join(' ');
+          classNameSet[styleSetArea] = classes.concat([registration.className]).join(' ');
         }
       } else {
-        // FIXME: classNameSet invalid types - exposed in TS 4.5 - cast needed
-        (classNameSet as Record<string, any>)[styleSetArea] = classes.join(' ');
+        classNameSet[styleSetArea] = classes.join(' ');
       }
     }
   }
diff --git a/packages/merge-styles/src/shadowConfig.ts b/packages/merge-styles/src/shadowConfig.ts
index 0cb6d1f8d647d..4a30066a471da 100644
--- a/packages/merge-styles/src/shadowConfig.ts
+++ b/packages/merge-styles/src/shadowConfig.ts
@@ -1,9 +1,10 @@
-export type ShadowConfig = {
+// eslint-disable-next-line @typescript-eslint/naming-convention
+export interface ShadowConfig {
   stylesheetKey: string;
   inShadow: boolean;
   window?: Window;
   __isShadowConfig__: true;
-};
+}
 
 export const GLOBAL_STYLESHEET_KEY = '__global__';
 export const SHADOW_DOM_STYLESHEET_SETTING = '__shadow_dom_stylesheet__';
@@ -13,7 +14,7 @@ export const DEFAULT_SHADOW_CONFIG: ShadowConfig = {
   inShadow: false,
   window: undefined,
   __isShadowConfig__: true,
-} as const;
+};
 
 export const makeShadowConfig = (stylesheetKey: string, inShadow: boolean, window?: Window): ShadowConfig => {
   return {
@@ -24,10 +25,14 @@ export const makeShadowConfig = (stylesheetKey: string, inShadow: boolean, windo
   };
 };
 
-export const isShadowConfig = (obj: unknown): obj is ShadowConfig => {
-  if (!obj) {
+export const isShadowConfig = (value: unknown): value is ShadowConfig => {
+  if (!(value && isRecord(value))) {
     return false;
   }
 
-  return (obj as ShadowConfig).__isShadowConfig__ === true;
+  return value.__isShadowConfig__ === true;
 };
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return value !== null && typeof value === 'object' && !Array.isArray(value);
+}