O'Malley played the back nine first at GardaGolf Country Club and stumbled out of the gate with a bogey at the 10th. He recovered nicely, however, with back- to-back birdies starting at the 11th and added a birdie at the 15th.
The Australian birdied the 18th to make the turn at 3-under before a remarkable stretch on his inward half. He birdied the third and chipped in from off the green for a birdie at the fourth.
At the par-5 fifth, O'Malley hit his second shot to 10 feet for an eagle. O'Malley kept rolling and drained a long putt for an eagle at the sixth.
O'Malley then knocked his approach inside six feet for a birdie at the seventh. He found trouble with a bogey at the following hole but managed to match the course-record established by Bernhard Langer in 1997.
'I've been working on my putting the last couple of days so it was good to come out and find that the greens were putting nicely,' said O'Malley, a three-time winner on the European Tour. 'I didn't come out with a lot of expectations, just hoping to play well, and I did.'
Clark also played the back half first and had a slow start with six straight pars. He caught fire at the 16th with the first of three consecutive birdies and picked up a pair of birdies from the second to surge up the leaderboard at 5-under.
The Englishman closed his bogey-free round with a birdie at the ninth, his last, for a 66.
'Last year was the first full season back for a couple of years after the injury and to be honest it has been recurring a little bit this week,' said Clark. 'It's all right playing but I feel it a little bit in practice, it's not too bad though.'
Argentina's Ricardo Gonzalez was one shot further back at 5-under-par 67. He was joined by Fredrik Orest of Sweden and England's Roger Chapman in a tie for third.
Stuart Little, Richard Sterne, Mattias Eliasson, Matthew Blackey, Tobias Dier, Pierre Fulke, Andrew Sherborne, Erol Simsek and David J. Geall finished tied for sixth at 4-under-par 68.
Colin Montgomerie, who is making his 2003 debut on the European Tour, played a steady round while getting accustomed to new clubs. Montgomerie carded a 70 to finish in a tie for 25th.
'I'll get into it a little bit more tomorrow afternoon once I relax a bit,' said Montgomerie. 'Obviously was a bit tense after the start I've made this year. I want to get going and sometimes I go too fast. But today was okay, it was steady, nothing wrong with that.'
Defending champion Ian Poulter struggled with three birdies, two bogeys and a triple-bogey to finish in a group at 2-over-par 74.