I saw this thread and didn't know to answer, but we had a hierarchical visit this Sunday and I served, so I think I can better answer. This comes from an OCA parish:
The bishop vested with the great omophor at the beginning of the service, and it was removed once he went to the High Place for the Epistle and Gospel reading. He was vested in the small omophor after his homily during the litany of the catechumens. It was removed again after the consecration of the Gifts at the end of the epiclesis.
I don't believe he wore it again for the rest of the service, though there are photos after the Liturgy (we had some gramotas and tonsurings, so they got photos with the bishop) and he's wearing the small omophor again. I don't recall when he put it back on, though. It might've been during the communing of the faithful, since he wasn't communing them (and I wasn't in the altar, so I would've missed it).