What Oberon-class submarines could do at Singapore in 1941

How dangerous would maintainable Oberon-class submarines be if based at Singapore in December 1941?

Extremely dangerous; they could inflict such severe losses that little of the Japanese invasion fleet might get through.

The premise gives a Second World War navy a much later conventional submarine and the ability to maintain it. An Oberon-class boat would gain major advantages in stealth, sensors, and underwater performance over contemporary anti-submarine defenses. A force of such boats operating from Singapore could attack the transports and escorts supporting Japan's December 1941 advance. The conclusion is deliberately counterfactual, but it shows how sharply mature postwar submarine design changed the balance between a submerged attacker and a wartime surface fleet.