Twelve months to the day since making his senior debut for the Black Cats as a substitute in O'Neill's first game in charge, a 2-1 win over Blackburn, the Ireland international fired them into a third-minute lead which they were never in danger of relinquishing.
Striker Steven Fletcher, whose return from an ankle injury gave his team an all together more threatening appearance, increased the advantage with an audacious 28th-minute backheel, before Stephane Sessegnon rounded Reading goalkeeper Adam Federici at the death to secure just a third league win in 24 attempts for Sunderland, and their first against 11 men since March.
O'Neill was adamant in the run-up to the rearranged fixture that it was not a must-win affair, and with owner and chairman Ellis Short a supportive presence at the stadium before kick-off, the club was presenting a united front.
However, a comfortable victory which lifted Sunderland out of the relegation zone was welcome nonetheless as some of the darker clouds hanging over the Stadium of Light dispersed.
The same cannot be said of newly promoted Reading, who remain locked in a battle for survival having won only one top-flight game, and who showed few signs of improving on that record on Wearside.
Short was a prominent presence in the press room before kick-off, ready to take issue with anyone who had the temerity to suggest his manager's reign could be drawing to a close.
McClean fired home from the edge of the box when a corner was punched to him, and the hosts pushed on from there.