Romero is joined atop the leaderboard by John Chillas, Jim Rhodes and 2002 U.S. Senior Open winner Don Pooley. Carl Mason and Ian Mosey share fifth place at 2-under-par 70.
Romero was steady to open his round. He parred the first and birdied the second. The Argentine then ran off sixth consecutive pars at Royal Portrush Golf Club.
Romero, an eight-time winner in the European Tour, dropped in back-to-back birdies from the ninth. He climbed to 4 under with a birdie at the 12th. Romero dropped a shot at the next, but came back to birdie the par-4 15th. He bogeyed the 16th to slide back into a share of the lead.
'I am very happy. It's my first tournament and I can see my old friends that I have known for a long time,' said Romero, who turned 50-years-old five days ago. 'I have never been here before, but Darren Clarke and Paul McGinley told me it was the best course in Northern Ireland. When I see the golf course and play it, I see that it is fantastic. It's very, very nice.'
Chillas spread one birdie and one bogey over his opening nine holes. He caught fire around the turn as he birdied the 10th. The Scotsman then drained consecutive birdies from the 12th to get to minus-3 before parring his final five holes.
Rhodes had a magical round. He birdied the first and eagled the second to quickly get to minus-3. The Englishman then dropped a stroke at the third.
The 58-year-old Rhodes aced the par-3 sixth to get to 4 under. Graham Marsh, who finished at plus-4, also had a hole-in-one at the sixth.
Rhodes' prosperity was short lived though as he bogeyed the very next hole. He picked up that lost stroke with a birdie at the par-5 10th. However, he dropped a stroke at 14th to slide back to 3 under.
Pooley struggled mightily out of the gate. He bogeyed three of his first four holes, however, Pooley did manage a birdie at the second. He parred his next four holes to head to the ninth at plus-2.
The 52-year-old Pooley then eagled the par-5 ninth. He came right back to eagle the par-5 10th to quickly jump to minus-2. Pooley birdied the 11th, but stumbled to a bogey on No. 12. He managed to grab a share of the lead with a birdie at the 13th. Pooley parred his final five holes to remain at minus-3.
Bruce Fleisher, Hugh Baiocchi, John Jacobs, Tom Kite and Bill Longmuir were the final players in red figures as they posted rounds of 1-under-par 71.
Tom Watson, the defending champion, posted an eagle, two birdies and seven bogeys in a round of 3-over-par 75. He is tied there for 42nd place.
'I drove it poorly three or four times today and it cost me,' Watson said. 'I deserved what I shot. I did hit a lot of good shots, but I didn't drive it well on the back nine. I had started the day hitting the ball pretty well. If three or four of those drives were better, it might have been a different story.'