Can you return a commander to the command zone after it's been exiled by Oblivion Ring?

In the Commander format, a replacement effect applies whenever your commander would be exiled: you may choose to put it into the command zone instead (CR 903.9). This is a replacement effect, meaning it changes where the card goes before it ever reaches exile. So when Oblivion Ring's enters-the-battlefield triggered ability would exile your commander, you redirect it to the command zone.

Critically, this has an important interaction with Oblivion Ring. O-Ring is designed so that if the exiled card ever stops being exiled (e.g., it leaves exile), O-Ring's leaves-the-battlefield trigger has nothing to return. Since your commander went to the command zone and never actually entered exile, if O-Ring later leaves the battlefield, its trigger will find no exiled card to return — the commander stays in the command zone (or wherever it has moved since).

This is governed by CR 903.9, which states that a commander may be moved to the command zone any time it would go to a graveyard or exile. It is also supported by CR 614.1, which covers replacement effects that redirect objects to different zones.

Example: Your opponent controls Oblivion Ring targeting your commander, Ur-Dragon. As O-Ring's trigger resolves, you apply the Commander replacement effect and put Ur-Dragon in the command zone. Later, Oblivion Ring is destroyed — its leaves-the-battlefield trigger tries to return the exiled card, but nothing is in exile, so nothing happens. Ur-Dragon remains in the command zone.

Note: The choice to redirect to the command zone is optional (CR 903.9). If for some reason you preferred your commander stay exiled (rare, but possible), you could allow it to go to exile normally instead.