UPDATE: In Episode 5, Yael Stone briefly shows her butt while her character is getting eaten out by Natasha Lyonne, but her hands stay over her breasts the entire time.

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Episode 1:

0:00 Laura Prepon (Brief Breasts) in Shower Kissing Taylor Schilling During Flashback(Brief Right Breast)
0:00 Taylor Schilling (Brief Butt from Side in Shower Stall)
0:01 Taylor Schilling (Breasts Stepping Out of Shower and Putting On Towel)

Episode 12:

0:09 Taryn Manning (Right Breast) in Bed With Man

Episode 13:
0:35 Taylor Schilling has the shower curtain pulled out and she covers up very quickly. Her breasts are briefly shown a few minutes later.

That seems to be it.