O'Connell sustained damage to his shoulder in the clash with France and missed the 31-5 victory over Wales but was expected to be given the all-clear to face Scotland.
However, O'Sullivan revealed on Tuesday he will give the British & Irish Lions forward another 48 hours to prove his fitness.
He said: "We're not sure about Paul and want to defer that decision until Thursday morning. I don't know what the chances of him playing are.
"We really thought he would make the Wales game but we found out on the Tuesday he wouldn't play. At the moment he's 50-50 and we need 48 hours more to decide.
"There's no point making the decision now. Paul trained [on Tuesday] but we have to be careful with how much work he does.
'It's an AC shoulder problem which has been slow healing."
If O'Connell is passed fit, Malcolm O'Kelly, Donncha O'Callaghan and Mick O'Driscoll will compete for the remaining place in the second row.
One will be named on the bench with the third - almost certainly O'Driscoll - dropping out of the squad altogether.
Prop John Hayes, struck down by a viral infection in the aftermath of the Wales match, retains his place at tighthead and is joined in the front row by Marcus Horan.
Horan was stretchered off late in the showdown with last season's Grand Slam champions following a collision with Gethin Jenkins, but his neck injury was only minor.
If O'Connell wins his fitness battle, he will be the solitary change to the starting line-up which dismantled Wales with O'Sullivan facing no other selection issues.
Ireland still have a magnificent chance of being crowned Six Nations champions with victory over Scotland setting-up a potential title decider against England on March 18.
Alongside England, who face a daunting trip to Paris on Sunday, Ireland are the only other team who still hold the destiny of the title in their own hands.
Even if France beat the world champions, their final match against Wales precedes Ireland's trip to Twickenham, potentially leaving O'Sullivan's men with a clear target for victory.
But first the Irish must dispatch Frank Hadden's rejuvenated Scotland who have emerged as contenders on the back of their shock wins over France and England.
Ireland team to face Scotland in the Six Nations Championship at Lansdowne Road on Saturday, March 11 is:
G Murphy (Leicester); S Horgan (Leinster), B O'Driscoll (Leinster, captain), G D'Arcy (Leinster), A Trimble (Ulster); R O'Gara (Munster), P Stringer (Munster); M Horan (Munster), J Flannery (Munster), J Hayes (Munster), M O'Kelly (Leinster)/P O'Connell (Munster)/D O'Callaghan (Munster)/M O'Driscoll (Munster), S Easterby (Llanelli), D Wallace (Munster), D Leamy (Munster).
Replacements: R Best (Ulster), S Best (Ulster), M O'Kelly (Leinster)/P O'Connell (Munster)/D O'Callaghan (Munster)/M O'Driscoll (Munster), J O'Connor (Wasps), E Reddan (Wasps), D Humphreys (Ulster), G Dempsey (Leinster).