A third Australian, Brett Rumford, shot a 6-under-par 66. He shares third place with Scotsman Stephen Gallacher and Englishman Simon Wakefield.
No Irishman has won this title since John O'Leary in 1982. Peter Lawrie is the top player from Ireland on the leaderboard as he stands at minus-5. He is joined in a tie for sixth by Fredrik Andersson, Robert-Jan Derksen, Sandy Lyle, Steve Webster and Mark Roe.
O'Hern began his day on the 10th tee at County Louth Golf Club. He parred his first two holes before dropping a stroke at the 12th. He bounced back to convert birdies at the 13th and 14th.
The 32-year-old moved to 2 under with a birdie at the par-4 16th. O'Hern caught fire as he hit the front nine. He dropped in a birdie at the first and came right back to birdie No. 2.
O'Hern made it three in a row as he birdied the third to get to minus-5. After a par at the fourth, the Australian picked up his sixth birdie at the par-3 fifth. He came right back with an eagle at the par-5 sixth to grab a share of the lead.
'This is the first time I have been here,' said O'Hern, who has four top-10 finishes in his last five starts. 'The course is fantastic. I love links golf where you have to think your way round and shape your shots. I was pretty annoyed I didn't get in the British Open last week at Royal Troon, I think that place would have suited me down to the ground. But I love this course and the firmness of it.'
Lonard bogeyed the 10th to open his round, but he came back to birdie three of four holes from the 12th to get to minus-2. He parred the final three holes of his opening nine, before birdieing the par-4 first.
The 20003 Australian Open winner dropped in back-to-back birdies from the third to get to minus-5. Like O'Hern, Lonard eagled the sixth. Lonard then closed with a birdie at the par-4 ninth to grab a share of the lead.
Lonard did not arrive until Wednesday evening. He was in London to get touch up work done on his eyes, which he had laser surgery on six years ago. With no complications from that surgery, he is giving it a go this week.
'I suppose when it is your day, it is your day,' Lonard said. 'The course is fantastic. I didn't know what to expect. I got here last night, so first look around I thought it was the greatest course I have ever played. It's a good links course.'
Michael Campbell opened his title defense with a 2-under-par 70 and stands in a tie for 28th.