Kassam told the BBC: "Ian has a contract until the end of the season. Between now and then I can extend it even if we don't get promoted, it's up to me.
"It's in my power to keep him for another year - I did explain that."
Bristol Rovers say that Atkins has accepted a two-year deal to take charge at the Memorial Ground.
Oxford are currently fourth in the Third Division, and pushing for a place in Division Two, while Rovers are three points above the relegation zone at the bottom of the table.