Can they do that ?

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It has not been documented.

Although this does NOT mean it cannot happen..


1 thing to take into consideration is that most oscars are very hard to sex. some say its impossible without venting, I am incline to agree.
 
Examining the fish's cloaca/urogenital region, which will provide concrete proof of the fish's sex.

Don't expect your oscar or any other common freshwater aquarium fish to change sexes on you...
 
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