Oxygen levels in iced-over water should remain sufficient for fish IF the ice is clear to allow sunlight through for aquatic plants/algae. There's minimal plant O2 production in cold water, but also minimal O2 consumption by fish. It balances.

If the ice is cloudy or snow covered, opening a hole through the ice (or removing at least 10% of the snow cover, which has never appealed to me personally) is warranted.