Poulter, who has been experiencing a dismal stretch of golf with three consecutive missed cuts coming into this event, brushed aside any problems he was having with four birdies on the front nine of the Wentwood Hills Course.
The Englishman continued his fine play on the inward half with a pair of birdies from the 10th. Poulter found trouble with a bogey at the par-3 14th but managed to respond at the very next hole.
Poulter hit his approach to 25 feet at the par-4 15th and confidently drained the putt for a birdie to return to 6-under. He closed his round with a four-foot birdie at the 18th for his share of first despite an ongoing battle with tonsillitis.
'Unfortunately I get this about five or six times a year which is why I always carry a stock of antibiotics with me so that I can take a five or six day course whenever I feel the symptoms coming on,' said Poulter, who posted his first sub-70 round since the Italian Open. 'But I am getting to the stage where I have had enough of this and I think this winter will be the time for me to get my tonsils out once and for all to save all this palaver.'
O'Hern, who has taken considerable time off this season, vaulted out of the gate with four birdies over his first five holes, including three straight starting at the par-4 second.
'I had four weeks rest and four weeks practice,' said O'Hern. 'But my wife is pregnant back in Australia with our first child which is due in August so my plan was always to travel back and forwards this year.'
The Australian cooled off until a birdie at the 13th. O'Hern pulled within one of the lead with a birdie at the 16th and added a birdie at the last to match Poulter at 7-under.
Jonathan Lomas also had a hot start with birdies on each of his first three holes. He added a birdie at the fifth to quickly reach 4-under but like O'Hern, it wasn't until late on his second nine that Lomas made birdie again.
The Englishman picked up back-to-back birdies starting at the 16th but only managed a par on the closing par-5 to finish one shot off the lead.
'I've been struggling a little bit but that's the best I've played for a long time,' said Lomas, who carded a 66. 'I've been working on trying to cut the ball in and managed to aim left today and actually get a cut on the ball.'
Peter Fowler, Alastair Forsyth, Jarmo Sandelin, Philip Archer, Richard Green, Ricardo Gonzalez and Malcolm Mackenzie finished tied for fourth at 5-under-par 67.
Local favorite Ian Woosnam, a top candidate to captain the European Ryder Cup squad in 2004, collected two birdies and two bogeys to finish in a group at even-par 72.
Colin Montgomerie was one shot further back at one-over-par 73.