JASON Davidson converted the 18th spot-kick of a nerve-jangling penalty shoot-out to book Albion’s passage into the Capital One Cup third round.
The Baggies should have wrapped the tie up in normal time but failed to convert their long spells of pressure into goals.
They only had the luckless Johnny Mullins’ 29th-minute own goal to show for their efforts, which was cancelled out by Danny Hylton’s dramatic equaliser four minutes from time.
League Two Oxford had chances to get their noses in front in extra-time and clung on bravely after Junior Brown was dismissed in the 111th minute for a second bookable offence.
It looked bleak for the Baggies on penalties when sub James Morrison and Brown Ideye missed their first two spot-kicks.
But Boaz Myhill came to Albion’s rescue by keeping out Michael Collins and Jake Wright’s penalties, paving the way for Davidson to secure a 7-6 spot-kick win.
Irvine handed full competitive debuts to summer signings Ideye, Davidson and Chris Baird as he made seven changes from Saturday’s goalless draw at Southampton.
Myhill, Claudio Yacob and Stephane Sessegnon also came in while Gareth McAuley saw his first action of the season following a calf problem.
Morrison also made a welcome comeback from a knee injury on the bench, where he was joined by new signing Cristian Gamboa.
The men to make way from the starting XI were benched trio Ben Foster, Jonas Olsson and Youssouf Mulumbu and injured quartet Craig Gardner, Graham Dorrans, Victor Anichebe and Sebastien Pocognoli.
The only change former Baggies player and coach Michael Appleton made to his Oxford team that lost 1-0 to Portsmouth on Saturday came in goal, where Max Crocombe replaced on-loan George Long, whose parent club Sheffield United denied him permission to play.
Albion took the game to Oxford, who sit at the foot of the league ladder having lost their opening four games, from the off.
U’s keeper Crocombe was called into action as early as the third minute when he kept out Chris Brunt’s unorthodox chest goalwards at point-blank range from Sessegnon’s centre.
Moments later, Sessegnon was off target with a first-time volley from Davidson’s ball inside.
The pressure continued and only Joe Riley’s last-ditch tackle denied Saido Berahino a clear sight of goal from Ideye’s neat lay-off.
Oxford grew into the game and enjoyed a mini spell of their own without troubling Myhill’s goal.
At the other end, Crocombe suffered a nervy moment when he fumbled Brunt’s deep centre. Ideye pounced on the rebound but his hook over the shoulder sailed wide.
Sessegnon then tried his luck from distance but could only find Crocombe’s midriff.
The United No.21 had a much tougher task keeping out Andre Wisdom’s rising 15-yard drive after the on-loan Liverpool man had powered into the box. The right-back’s shot was too hot for Crocombe to hold but Oxford scrambled the ball clear.
A goal had been coming, though, and it arrived on 29. Wisdom fizzed a vicious centre into the box which clipped Tom Newey before flying into the net off unsighted defensive colleague Mullins’ shins.
Crocombe must have felt like a shooting target as another Sessegnon effort came his way. He was unable to hold the 20-yard strike but gathered at the second attempt.
Sessegnon combined brilliantly with Ideye on 34 before the latter fizzed a terrific 18-yard shot on the turn just over.
That proved the final action of a half Albion had dominated.
The hosts picked up where they left off after the break and a sweeping move resulted in Ideye producing an air-shot at the near-post form Wisdom’s cross.
Yacob made a much better connection with a free back-post header from Brunt’s 56th-minute corner – but the midfielder could only direct it straight at Crocombe.
Albion lost possession in their own half on 63 and it took a terrific last-ditch challenge from McAuley to prevent Josh Ruffels troubling Myhill’s goal.
The Baggies surged straight upfield and Crocombe had to produce a smart near-post save to keep out Sessegnon’s volley from Davidson’s tee-up.
There was a welcome site for the home hordes on 64 when Morrison returned to action for the first time this season in place of Brunt.
Despite putting the United goal under concerted pressure, Albion had not killed this tie off and Oxford almost levelled on 66.
The lively Brown played in Carlton Morris and the United No.9 dragged his 12-yard strike agonisingly wide of the beaten Myhill’s far post.
Roared on by an impressive away following, United’s confidence was growing by the minute.
But Albion eventually regained the initiative and Sessegnon saw a low drive deflected into the side-netting.
However, there was late drama as United squared the tie with four minutes to go. Sub Will Hoskins headed a deep left-wing centre across goal and Hylton bundled the ball home.
Both teams pushed for a winner and Crocombe did well to keep out Berahino’s near-post effort after good work from Ideye in the third minute of stoppage time.
Moments later, Davidson powered into the box from the left but could only shoot straight at the Oxford keeper from a tight angle. Cue extra-time.
With less than two minutes on the clock, Morrison collected a Wisdom pass and saw a fierce 15-yard drive deflected just wide.
But Oxford went even closer to adding a second. After Myhill fumbled a low cross, Craig Dawson had to display sheer brute strength to prevent Brown scrambling home the rebound before clearing.
Moments later, Brown was presented with another chance but could only fire straight at the alert Myhill.
During the half-time of extra-time, Irvine made his final throw of the dice replacing Chris Baird with debutant Gamboa.
Oxford started the final 15 minutes brightly and Myhill had to scamper across his line to keep out sub Alex Jakubiak’s scuffed shot.
But the visitors were reduced to ten men in the 111th minute when Brown was shown a second yellow card for a late challenge on Dawson.
Albion pushed for a winner and Wisdom was only a whisker away with a terrific 20-yard drive in the 118th minute.
Dawson had the ball in the net in the 120nd minute but his header from Morrison’s corner was ruled out for a foul, which sentenced the game to a nerve-wracking penalty shootout in front of the Brummie Road End.
0-1: OXFORD – Hylton scores; ALBION – Morrison’s pen saved by Crocombe
0-1: OXFORD – Myhill saves Collins’ spot-kick; ALBION – Crocombe keeps out Ideye’s pen
1-1: OXFORD – Newey fires wide; ALBION – Mulumbu scores
2-2: OXFORD – Hoskins scores; ALBION – Berahino scores
3-3: OXFORD – Hunt scores; ALBION – Yacob scores
4-4: OXFORD – Jakubiak scores; ALBION – Dawson scores
5-5: OXFORD – Ruffels scores; ALBION – Wisdom scores
6-6: OXFORD – Mullins scores; ALBION – Gamboa scores
7-6: OXFORD – Myhill denies Wright; ALBION – Davidson scores
ALBION (4-2-3-1): Myhill; Wisdom, Dawson, McAuley, Davidson; Yacob, Baird (Gamboa ET-HT); Berahino, Sessegnon (Mulumbu 78), Brunt © (Morrison 64); Ideye. Subs not used: Foster (gk), Olsson, O'Neil, Roofe.
OXFORD (4-4-2): Crocombe; Riley (Hunt HT), Mullins, Wright ©, Newey; Rose (Jakubiak 70), Collins, Ruffels, Brown; Morris (Hoskins 81), Hylton. Subs not used: Stephens (gk), Meades, Raynes, Potter.
GOALS: ALBION – Mullins (own goal, 29). OXFORD – Hylton (86).
BOOKINGS: OXFORD – Brown (foul 52), Wright (foul 116), Crocombe (time-wasting 118).
SENDING-OFF: OXFORD – Brown (foul, second bookable offence 111).
REFEREE: J Adcock (Nottinghamshire).
ALBION STARMAN: Andre Wisdom.