Ochoa -- ranked No. 1 in the world -- finished 54 holes at 13-under-par 203 and is one stroke ahead of Prammanasudh, who bogeyed her last hole to fall out of a share of first and a 1-over 73.
Virada Nirapathpongporn and Maria Hjorth both had 70s on Saturday and are tied for third place at 207.
Prammanasudh took a three-shot lead into Saturday's third round.
Ochoa has six victories so far this year and has won her last three starts.