Sessegnon is believed to have struggled to settle in the North East with his family still living in France.
The news has alerted a number of Ligue 1 clubs to Sessegnon's possible availability in this month's transfer window.
O'Neill is aware of the off-field issues, but the Black Cats chief hopes the former Paris Saint Germain forward will remain at Sunderland.
"That is obviously a problem," said O'Neill. "But it doesn't seem to have affected him on the field because his form has been excellent. I hope he gets the chance to genuinely settle here, but it is not easy.
"I understand, but it hasn't disturbed him and he hasn't come to me and said it has been a major problem.
"He's got great ability and I would love him to settle here and make this his home."