Play is contested using the Modified Stableford Scoring system. The system offers eight points for a double eagle, five points for an eagle and two points for a birdie. No points are handed out for par, and one point is deducted for each bogey with players losing three points for double bogey or worse.
Laura Davies, a star on the LPGA Tour, struggled on Thursday with five bogeys and two birdies for a minus-1 total. She is tied for 124th place.
Davies, a 20-time winner on the LPGA Tour and a veteran of all seven Solheim Cups, acknowledged that nerves were a big factor in her difficult opening round.
'The trouble is, for these guys it's just an ordinary event for them and they're just trying to win it and I'm out there under all sorts of pressure,' admitted Davies. 'If I'm honest, I was just scared of making a fool of myself all the way round until the last few holes.'
Davies started on the back nine and tallied five bogeys in her first 11 holes. She rebounded with birdies at the par-5 fourth and the par-3 eighth to have a shot at making the 36-hole cut with a good second round on Friday.
'Because the chances were there and it gives me encouragement tomorrow if I can cut the rubbish out and just make four, five birdies, then you never know what's going to happen,' she said.
John Bickerton is alone in second place with 16 points, followed by Steve Webster and U.S. Amateur champion Nick Flanagan, who share third place with 14 points.
O'Hern began Thursday's opening round on the back nine at Horizons and birdied the 11th hole. At the par-5 12th, O'Hern chipped in for eagle from a little farther than 30 yards, then chipped in from 20 yards out for an eagle at the par-5 16th.
'I really lit it up on the front nine,' O'Hern said. 'I got away to a great start with a birdie on 11, and chipped in on 12 for eagle. Funnily enough I lipped out on the next for a chip-in as well and then chipped in again for an eagle.'
He birdied the 18th hole to finish off his front nine with 14 points.
On the second nine, O'Hern played steady golf with six consecutive pars. He recorded birdies at the seventh and eighth holes to gain four more points. O'Hern narrowly missed the 18-hole point record by two points and credited the format for his aggressive play.
'Today I just went at every flag,' said O'Hern. 'I found out the first year I played this format that's what you have to do. You can't afford to make a lot of pars and one or two birdies, that's just not going to get you anywhere. If you make a bogey it's only a one-point loss and doesn't matter too much.'
Bickerton was on fire on the front nine with six birdies, but cooled off on the second nine. He racked up birdies at 11 and 13, but played the final five holes at even-par to take a solo second.
Peter Hedblom is alone in fifth place with 13 points, followed by Peter O'Malley, Mark Foster and Joakim Haeggman, who are knotted in sixth place with 12 points.