Three players are tied for third at 7-under 137 including two-time Masters champion Nick Faldo (69). The Englishman is joined by American Mike Cunning (70) and Charlie Wi of Korea (67).
O'Hern, who fired a course-record 8-under 64 to grab the lead in the opening round, birdied both of the par-fives on the front nine at the Laguna National Golf and Country Club to climb to 10-under.
After opening a three-stroke advantage over his closest competitors, he cooled down with eight straight pars until a bogey on the par-4 16th dropped him to 9-under.
'I played pretty similar to yesterday but just didn't hole the putts,' said O'Hern. 'It was pretty decent but I started to struggle over the last four or five holes and threw in a bogey.'
Despite his frustrations, the 30-year-old managed to come out on top.
'Tomorrow you want to put yourself in position to have a chance on Sunday,' said O'Hern. 'If I am in the lead you have a chance to increase it. If not as long as I am not far away going in to Sunday, especially on the back nine, then that is where you want to have your opportunity to finish on top.'
Johnson, a journeyman professional who has played in South Africa and South America, had seven birdies and two bogeys to stand alone in second.
'I didn't hit a lot of big shots and I just kept it in play and kept it on the right side of the hole and made a few good putts,' said Johnson, who was the winner at the Asian PGA Qualifying School last month in Malaysia. 'It's the kind of course you can make birdies on.'
Faldo had four birdies and a bogey on his round to put himself in contention. The 44-year-old, who has not won since the 1997 Nissan Open, has played good golf this year, including a top-10 finish at the Heineken Classic.
'I feel like I am getting into a bit of a groove,' said Faldo. 'I am still working hard and haven't got the confidence in the swing yet but it is coming. I am just trying to push myself to gain confidence from the good shots I hit. It's a harder game now ten years on. I really have to boost myself up and make myself go for it.'
The English duo of Gary Clark (66) and Chris Williams (71) finished three shots behind O'Hern. They were joined by Sweden's Christopher Hanell (69), American Ted Purdy (69), Taiwan's Wang Ter-chang (71) and Carlos Rodiles (72) of Spain at 6-under 138.
The cut fell at 1-under 143 with 71 players missing the weekend. Among those was 1991 Masters champion Ian Woosnam.
This event is co-sanctioned by the European and Davidoff Tours.
Full-field scores from the Singapore Masters