The South Korean striker was brought to Wearside from his native land last summer under the reign of former Black Cats manager Steve Bruce.
However Ji has failed to establish himself as a regular this term and his future has been in doubt since O'Neill's appointment in December.
"We've had a few phone calls from clubs abroad that would like to take Ji on loan, but we've knocked them back," O'Neill told the Sunderland Echo.
"The fact is, we can't afford to lose any of our squad at the moment, but besides that, Ji is a player who has done well for us.
"The lad has talent, but he's just come over here and is still learning the game. We want to see how he progresses and develops."
Ji's only Sunderland goal to date was a stoppage-time winner against Manchester City at the Stadium of Light over the festive period.