The three are 6-under-par 138 through 36 holes.
Woods shot 4-under 68, Oberholser shot his second straight 3-under 69 and Singh came in with a 1-under 71 Friday.
"I got off to a pretty good start this morning, Woods said."(But) I felt like I pretty much have maximized my rounds."
"I couldn't get the greens this morning, I don't know why," Singh said.
Ted Purdy (69) and Jason Bohn (72) are tied for fourth place at 5-under 139.
A bogey-triple bogey ending cost Harrington a shot at the leaders.