Sources on Wearside had insisted earlier that O'Neill was safe regardless of what happened on Tuesday evening but, as the first half unfolded, it swiftly became clear Reading were no threat to the former Aston Villa manager's job security.
It took O'Neill's side less than three minutes to take the lead when Royals keeper Adam Federici could only punch Adam Johnson's inswinging corner to Republic of Ireland winger McClean and he fired home from the edge of the area.
Sebastian Larsson almost doubled Sunderland's lead within two minutes as his shot was beaten away by Federici.
The hosts doubled their lead in the 28th minute when full-back Danny Rose surged forward and delivered a low cross to the near post, where Scotland striker Fletcher produced a deft flick past Federici.
Johnson, who had earlier been felled by Jobi McAnuff with a crude challenge, failed to reappear after the break and was replaced by Phil Bardsley. But Sunderland remained in control despite that loss.
Federici had to make saves from McClean and Craig Gardner to keep the score down but, deep into stoppage time, Sessegnon applied the finishing touch to Carlos Cuellar's long ball to seal a rare rewarding night for O'Neill.
-AAP