All she had to do was top the No. 1 player in the game in a playoff.
Castrale managed a 1-under 71 on Sunday while Ochoa could do no better than a 2-over 74. The pair finished regulation at 9-under-par 279 and headed back to the 18th at RiverTowne Country Club for the playoff.
Ochoa went first and badly pulled her tee shot into a hazard. Castrale split the fairway, then conservatively played to the center of the green, some 30 feet from the hole. Ochoa had to take a penalty drop, but hit a spectacular third shot 18 feet short of the stick.
Castrale cozied her birdie putt just right of the hole, then tapped in for par. Ochoa needed to hole her putt to extend the playoff, but the ball skimmed the left edge, but stayed above ground.
The title was Castrale's first and as is customary on the LPGA Tour, the beer and champagne shower followed thanks to Laura Diaz and Cristie Kerr.
'I don't know if it's quite hit me yet,' admitted Castrale, who pocketed $390,000 for the win. 'It was fun out there. I stayed patient and it worked out.'
Kerr was in the mix until a disastrous double-bogey at the 17th. She shot a one-over 73 for third place at minus-6.
Paula Creamer and Sarah Lee both shot rounds of 1-under 71 on Sunday to tie for fourth place at 5-under-par 283. Tour rookie Angela Park struggled to a 4-over 76 and took sixth at minus-3.
Castrale and Ochoa reached the playoff due to a combination of clutch putting from Castrale and some poor work by the No. 1 ranked player in the Rolex Rankings.
Castrale was solid throughout Sunday's final round with a bogey at the second and a 40-foot birdie putt at the sixth. Her only other birdie came at the par-5 11th until an important one later in the round.
Ochoa, who started the round with a three-shot lead, parred her first six holes before a bogey at the seventh. She rebounded with a 10-foot birdie putt at the eighth, which reclaimed her three-shot edge.
Ochoa lost a stroke with a bogey at the par-5 11th. It became a two-shot swing thanks to Castrale's birdie at the same hole, but Ochoa, now only one in front, gave herself some more chances.
She missed a 5-foot birdie putt at the 14th, but rolled in a birdie putt at the 15th from 12 feet.
Both players bogeyed the par-5 16th, so Ochoa walked to the 17th tee with a two-shot lead. Ochoa's nine-iron tee ball landed on the back fringe and she hammered her second 7 feet past.
Castrale spun her tee ball back to 12 feet and canned the birdie try to get within one. Ochoa missed her par putt and now the pair was tied with one to play.
'I knew I had to make that,' acknowledged Castrale. 'I expected Lorena to make par from there. Unfortunately she hit a good putt and it didn't go in. I made birdie.'
The two made somewhat tricky par saves at the 18th in regulation, then Castrale walked off with the victory.
Christina Kim (70), Heather Young (71), Mi Hyun Kim (73) and Angela Stanford (75) tied for seventh place at 2-under-par 286.
Tournament host Annika Sorenstam, who returned to action this week after missing time with a neck and back injury, shot a 4-over 76 on Sunday and tied for 36th place at plus-5.