The O'Neill Effect has seen Sunderland climb to ninth in the Premier League table, and they knocked Arsenal out of the FA Cup last weekend, but they came unstuck against a West Brom side who had previously won only two games at home all season.
Nigerian forward Peter Odemwingie, who scored a hat-trick in the recent 5-1 demolition of Wolverhampton Wanderers, broke the deadlock with an early goal and James Morrison added a second four minutes before half-time.
Odemwingie took his tally to five goals in two games with the hosts' third at the start of the second half, before Keith Andrews completed a victory that lifted Roy Hodgson's men to 12th in the table.
Sunderland's resurgent form has catapulted them into the top half but they fell behind after only three minutes at The Hawthorns when Odemwingie stole between two defenders to head home Youssef Mulumbu's driven cross from the left.
Recent Ireland call-up James McClean squandered an opportunity to level when he failed to capitalise on an inviting Sebastian Larsson cross, before Morrison tested Simon Mignolet at the other end with a rasping shot from the edge of the box.
The Scotland midfielder would not be denied a second time, however, and on 41 minutes he planted Jerome Thomas' centre past Mignolet with a downward back-post header to double the hosts' advantage.
O'Neill introduced Nicklas Bendtner and new England call-up Fraizer Campbell at half-time but any hopes of a Black Cats comeback were shattered within three minutes of the second period.
West Brom goalkeeper Ben Foster launched a counter-attack with an intelligent long throw that allowed Morrison to feed Marc-Antoine Fortune, whose lay-off eluded Morrison but was dispatched in crisp fashion by Odemwingie.
Thomas curled a shot against the crossbar moments later, and Fortune had an effort cleared off the line, while Sunderland's best chance saw Michael Turner head over from Wayne Bridge's free-kick.
Andrews completed O'Neill's misery in the last minute, burying a Graham Dorrans cut-back and sealing Sunderland's heaviest Premier League defeat since a 5-0 reverse at Manchester City last April.
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