O'Hair fired a 2-under-par 68 to finish 72 holes at 4-under 276, ending up into a tie with Blanks, who had carded a 69.
The pair returned to the 18th hole for the playoff, where Blanks put his second shot from the rough into a greenside bunker and O'Hair, who had also found the rough, played back into the fairway with his second.
O'Hair then chipped 25 feet short of the flagstick, but Blanks overshot the green, finding the fringe. O'Hair just missed a par putt, settling for a bogey tap-in.
Blanks, seeking his first PGA title, needed to chip into the hole for the win but couldn't, instead running 5 feet past the cup. He then missed right and the title was O'Hair's -- the fourth of his PGA Tour career.
Andres Romero shot an even-par 70 and just missed the playoff by a stroke at 3-under 277.
Canadian Adam Hadwin (72) tied for fourth place with former U.S. Open champion Geoff Ogilvy (70) at minus-2.
Swim Week Miami Beach 2014 [PHOTOS]