Thompson was red carded with telve minutes to go in today's game with Aberdeen following a tussle with the Dons frontman Zeroali.
O'neill had this to say about the dismissal:
"Alan Thompson hasn't said too much in the dressing room," said O'Neill.
"But he is obviously disappointed that he'll miss the Rangers game.
"It's a shame because he played very well and took the penalty kick for us.
"Seemingly it was a nothing incident and I thought he'd learn from Monday
night with John Hartson so it's very disappointing.
"I'll have to have a look at it tonight and take action accordingly."
O'Neill must now be hoping Henrik Larsson is fit enough to return to first team action as Celtic home in on the league title.