What happens to a creature enchanted with Oblivion Ring if Oblivion Ring leaves the battlefield before its second trigger resolves?

Oblivion Ring has two separate triggered abilities: one that exiles a nonland permanent when O-Ring enters the battlefield, and one that returns the exiled card when O-Ring leaves the battlefield. These are independent triggers that go on the stack separately.

The classic 'blink trick' works as follows: if Oblivion Ring's first trigger (exile) is still on the stack and you cause O-Ring to leave the battlefield before that trigger resolves, the second trigger (return) goes on the stack above the first. The return trigger resolves first — but there is nothing yet exiled, so nothing returns. Then the exile trigger resolves and exiles the target permanently, with no O-Ring on the battlefield to ever trigger a return. Per CR 603.6, a triggered ability checks the game state only when it resolves; since no card is exiled when the return trigger resolves, it does nothing.

If instead O-Ring's first trigger has already resolved (the creature is already in exile) and then O-Ring leaves the battlefield, the return trigger goes on the stack normally and the creature will come back when that trigger resolves. You cannot strand the creature in exile at that point just by removing O-Ring.

Concrete example: You cast Oblivion Ring targeting your opponent's Baneslayer Angel. In response, your opponent uses Flickerwisp to blink your O-Ring. The return trigger (nothing to return yet) is placed on the stack on top of the exile trigger. Return trigger resolves — no exiled card, nothing happens. Exile trigger resolves — Baneslayer Angel is exiled. With O-Ring gone, it stays in exile forever.

Relevant rules: CR 603.2 (triggered abilities trigger when their condition is met), CR 603.3 (triggered ability goes on the stack at next opportunity), and CR 400.7 (objects in exile are in a separate zone; the ability tracking the exiled card relies on O-Ring remaining on the battlefield to generate the return trigger).