diff --git a/fastapi/security/oauth2.py b/fastapi/security/oauth2.py
index 42674b476c..d497ee732c 100644
--- a/fastapi/security/oauth2.py
+++ b/fastapi/security/oauth2.py
@@ -398,6 +398,13 @@ class OAuth2(SecurityBase):
         self.scheme_name = scheme_name or self.__class__.__name__
         self.auto_error = auto_error
 
+    def _set_flows(
+        self,
+        *,
+        flows: OAuthFlowsModel | dict[str, dict[str, Any]],
+    ) -> None:
+        cast(OAuth2Model, self.model).flows = cast(OAuthFlowsModel, flows)
+
     def make_not_authenticated_error(self) -> HTTPException:
         """
         The OAuth 2 specification doesn't define the challenge that should be used,
@@ -452,7 +459,7 @@ class OAuth2PasswordBearer(OAuth2):
                 [FastAPI docs for Simple OAuth2 with Password and Bearer](https://fastapi.tiangolo.com/tutorial/security/simple-oauth2/).
                 """
             ),
-        ],
+        ] = "",
         scheme_name: Annotated[
             str | None,
             Doc(
@@ -514,6 +521,21 @@ class OAuth2PasswordBearer(OAuth2):
             ),
         ] = None,
     ):
+        super().__init__(
+            flows=OAuthFlowsModel(),
+            scheme_name=scheme_name,
+            description=description,
+            auto_error=auto_error,
+        )
+        self.initialize(tokenUrl=tokenUrl, refreshUrl=refreshUrl, scopes=scopes)
+
+    def initialize(
+        self,
+        *,
+        tokenUrl: str,
+        refreshUrl: str | None = None,
+        scopes: dict[str, str] | None = None,
+    ) -> None:
         if not scopes:
             scopes = {}
         flows = OAuthFlowsModel(
@@ -526,12 +548,7 @@ class OAuth2PasswordBearer(OAuth2):
                 },
             )
         )
-        super().__init__(
-            flows=flows,
-            scheme_name=scheme_name,
-            description=description,
-            auto_error=auto_error,
-        )
+        super()._set_flows(flows=flows)
 
     async def __call__(self, request: Request) -> str | None:
         authorization = request.headers.get("Authorization")
@@ -552,7 +569,7 @@ class OAuth2AuthorizationCodeBearer(OAuth2):
 
     def __init__(
         self,
-        authorizationUrl: str,
+        authorizationUrl: str = "",
         tokenUrl: Annotated[
             str,
             Doc(
@@ -560,7 +577,7 @@ class OAuth2AuthorizationCodeBearer(OAuth2):
                 The URL to obtain the OAuth2 token.
                 """
             ),
-        ],
+        ] = "",
         refreshUrl: Annotated[
             str | None,
             Doc(
@@ -619,6 +636,27 @@ class OAuth2AuthorizationCodeBearer(OAuth2):
             ),
         ] = True,
     ):
+        super().__init__(
+            flows=OAuthFlowsModel(),
+            scheme_name=scheme_name,
+            description=description,
+            auto_error=auto_error,
+        )
+        self.initialize(
+            authorizationUrl=authorizationUrl,
+            tokenUrl=tokenUrl,
+            refreshUrl=refreshUrl,
+            scopes=scopes,
+        )
+
+    def initialize(
+        self,
+        *,
+        authorizationUrl: str,
+        tokenUrl: str,
+        refreshUrl: str | None = None,
+        scopes: dict[str, str] | None = None,
+    ) -> None:
         if not scopes:
             scopes = {}
         flows = OAuthFlowsModel(
@@ -632,12 +670,7 @@ class OAuth2AuthorizationCodeBearer(OAuth2):
                 },
             )
         )
-        super().__init__(
-            flows=flows,
-            scheme_name=scheme_name,
-            description=description,
-            auto_error=auto_error,
-        )
+        super()._set_flows(flows=flows)
 
     async def __call__(self, request: Request) -> str | None:
         authorization = request.headers.get("Authorization")
diff --git a/tests/test_security_oauth2_lazy_initialization.py b/tests/test_security_oauth2_lazy_initialization.py
new file mode 100644
index 0000000000..0c0d8fa72d
--- /dev/null
+++ b/tests/test_security_oauth2_lazy_initialization.py
@@ -0,0 +1,86 @@
+# Ref: https://github.com/fastapi/fastapi/issues/3317
+
+from fastapi import APIRouter, FastAPI, Security
+from fastapi.security import OAuth2AuthorizationCodeBearer, OAuth2PasswordBearer
+from fastapi.testclient import TestClient
+
+auth_code_scheme = OAuth2AuthorizationCodeBearer()
+auth_code_router = APIRouter()
+
+
+@auth_code_router.get("/private-route")
+async def private_route(
+    token: str | None = Security(auth_code_scheme, scopes=["admin"]),
+):
+    return {"token": token}
+
+
+def create_auth_code_app() -> FastAPI:
+    app = FastAPI()
+    app.include_router(auth_code_router)
+    auth_code_scheme.initialize(
+        authorizationUrl="https://example.com/authorize",
+        tokenUrl="https://example.com/oauth/token",
+        scopes={"admin": "Admin access"},
+    )
+    return app
+
+
+def test_oauth2_authorization_code_bearer_lazy_initialize():
+    app = create_auth_code_app()
+    client = TestClient(app)
+
+    response = client.get(
+        "/private-route", headers={"Authorization": "Bearer testtoken"}
+    )
+    assert response.status_code == 200, response.text
+    assert response.json() == {"token": "testtoken"}
+
+    openapi = client.get("/openapi.json")
+    assert openapi.status_code == 200, openapi.text
+    authorization_code_flow = openapi.json()["components"]["securitySchemes"][
+        "OAuth2AuthorizationCodeBearer"
+    ]["flows"]["authorizationCode"]
+    assert (
+        authorization_code_flow["authorizationUrl"] == "https://example.com/authorize"
+    )
+    assert authorization_code_flow["tokenUrl"] == "https://example.com/oauth/token"
+    assert authorization_code_flow["scopes"] == {"admin": "Admin access"}
+
+
+password_scheme = OAuth2PasswordBearer()
+password_router = APIRouter()
+
+
+@password_router.get("/password-route")
+async def password_route(token: str | None = Security(password_scheme)):
+    return {"token": token}
+
+
+def create_password_app() -> FastAPI:
+    app = FastAPI()
+    app.include_router(password_router)
+    password_scheme.initialize(
+        tokenUrl="https://example.com/oauth/token",
+        scopes={"read": "Read access"},
+    )
+    return app
+
+
+def test_oauth2_password_bearer_lazy_initialize():
+    app = create_password_app()
+    client = TestClient(app)
+
+    response = client.get(
+        "/password-route", headers={"Authorization": "Bearer testtoken"}
+    )
+    assert response.status_code == 200, response.text
+    assert response.json() == {"token": "testtoken"}
+
+    openapi = client.get("/openapi.json")
+    assert openapi.status_code == 200, openapi.text
+    password_flow = openapi.json()["components"]["securitySchemes"][
+        "OAuth2PasswordBearer"
+    ]["flows"]["password"]
+    assert password_flow["tokenUrl"] == "https://example.com/oauth/token"
+    assert password_flow["scopes"] == {"read": "Read access"}