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F/O usually stands for Fictional Other, which is a term often used by Self-shippers, mainly on Tumblr.

You can have romantic, familial and platonic F/Os!
Self Shipper 1#: Yo, I got a new F/O!!
Self-Shipper 2#: That's so cool, who are they?
Self Shipper 1#: It's Peter Parker from The Amazing Spider-Man, 2012!
by enotits October 14, 2018
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