The halfway leader from South Africa struck a beautiful tee shot on the par three, but saw his ball lip out leaving a straight-forward birdie putt.
An ace would have taken Goosen to nine under, two off the lead with two remaining, but although the twice US Open Championship winner was on course to set the clubhouse target, Mahan and O'Hair were well clear at 11 under.
The American pair had two more compatriots, Bo van Pelt and Ryan Palmer, leading the chase at nine under as a tense finish brewed at Firestone.
Swede Peter Hanson was the leading European at seven under as he reached the turn, alongside Jim Furyk and Matt Kuchar.
Phil Mickelson was set to wait a little longer before taking Tiger Woods' World Number One spot - an outward 41 all but ending his chances of the top four finish required to overhaul Woods.