The 26-year-old South African totaled 12-under 132 at Doha Golf Course, overtaking first-round co-leader leader Coltart of Scotland (69). Alvaro Quiros of Spain (67) and Brett Rumford of Australia (69) were another stroke back.
Overnight co-leader Miguel Angel Jimenez of Spain (71) was in fifth place.
I am pleased with my game, Oosthuizen said. The only dodgy moment came when I thought I would bogey the fifth hole, but I got out of it. I had a tough chip on the fifth, tried to be cute, but luckily I didnt bogey that. After that, I played regulation golf, good putts.
Coltart, the inaugural 1998 Qatar Masters champion, gathered his birdies despite heavy morning rain on the course. Quiros also had seven birdies' including three straight to end the round'to offset bogeys on the 2nd and 15th.
Sergio Garcia of Spain (70) was eight strokes behind the leader, while defending champion Adam Scott of Australia (70) was 11 shots back.
Paul Casey of England, who won the Abu Dhabi Championship earlier this month, missed the cut, along with Darren Clarke, Colin Montgomerie and Justin Rose.