diff --git a/.changeset/bright-pianos-divide.md b/.changeset/bright-pianos-divide.md
new file mode 100644
index 000000000..893f63f71
--- /dev/null
+++ b/.changeset/bright-pianos-divide.md
@@ -0,0 +1,5 @@
+---
+"mobx": patch
+---
+
+fix #3595 onBecomeObserved is not called for ObservableSet
diff --git a/packages/mobx/__tests__/v5/base/become-observed.ts b/packages/mobx/__tests__/v5/base/become-observed.ts
index d643ab63d..32d42d346 100644
--- a/packages/mobx/__tests__/v5/base/become-observed.ts
+++ b/packages/mobx/__tests__/v5/base/become-observed.ts
@@ -509,3 +509,20 @@ test("#2667", () => {
         "onBecomeUnobservednew"
     ])
 })
+
+test("works with ObservableSet #3595", () => {
+    const onSetObserved = jest.fn()
+    const onSetUnobserved = jest.fn()
+
+    const set = observable.set()
+
+    const disposeOBO = onBecomeObserved(set, onSetObserved)
+    const disposeOBU = onBecomeUnobserved(set, onSetUnobserved)
+    const diposeAutorun = autorun(() => set.size)
+    diposeAutorun()
+    disposeOBO()
+    disposeOBU()
+
+    expect(onSetObserved).toBeCalledTimes(1)
+    expect(onSetUnobserved).toBeCalledTimes(1)
+})
diff --git a/packages/mobx/src/types/observableset.ts b/packages/mobx/src/types/observableset.ts
index 05171775a..16c797b4f 100644
--- a/packages/mobx/src/types/observableset.ts
+++ b/packages/mobx/src/types/observableset.ts
@@ -65,7 +65,7 @@ export type ISetWillChange<T = any> =
 export class ObservableSet<T = any> implements Set<T>, IInterceptable<ISetWillChange>, IListenable {
     [$mobx] = ObservableSetMarker
     private data_: Set<any> = new Set()
-    private atom_: IAtom
+    atom_: IAtom
     changeListeners_
     interceptors_
     dehancer: any
diff --git a/packages/mobx/src/types/type-utils.ts b/packages/mobx/src/types/type-utils.ts
index a85dd327d..161ca8bc3 100644
--- a/packages/mobx/src/types/type-utils.ts
+++ b/packages/mobx/src/types/type-utils.ts
@@ -22,7 +22,7 @@ export function getAtom(thing: any, property?: PropertyKey): IDepTreeNode {
             return (thing as any)[$mobx].atom_
         }
         if (isObservableSet(thing)) {
-            return (thing as any)[$mobx]
+            return thing.atom_
         }
         if (isObservableMap(thing)) {
             if (property === undefined) {