After parring the first five holes, Oh birdied holes six and seven and then added birdies on 10, 13 and 17 to take a one stroke lead over seven players including England's Lee Westwood. The 2000 Order of Merit champion on the European Tour was without a bogey during his round of 68.
Oh, who became the youngest player ever to qualify for the U.S. Open as a 16-year-old amateur in 1993, has struggled this year at 94th on the Merit list and has made just seven cuts in 14 starts.
American John Daly and Colin Montgomerie of Scotland are three shots off the pace following two-under 70s which featured four birdies and two bogeys in their rounds.
Paul McGinley of Ireland, American Bob May and Scottsman Andrew Coltart are at one-under par while Michael Campbell of New Zealand struggled to a one-over 73.
Davidoff Tour Order of Merit leader Arjun Atwal of India carded a 74. Ten players failed to finish their rounds due to darkness.
This is the first year of the event which features a total purse of $1,000,000 with a first-place check of $161,450.
Harbour Plaza Golf Club is a Robert Trent Jones II design which plays to a par of 72, stretching to 7,084 yards.
Leaderboard from the TCL Classic