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Other significant other is a term employed in the polyamorous community in more than one way. It can be used to refer to one's partner's partner. ("He's my boyfriend's other significant other.")
Additionally, it can be used to refer to one's own partners. Sometimes this will be done to refer to partners outside of a primary partnership, but it may also be done as a simple means of identifying multiple partners.
The term other significant other can be a bit clunky and is sometimes shortened to OSO. In reference to partner's partners, the term metamour is often employed in the same way as other significant other.