F/O stands for “Fictional Other”, like “significant other” can be used to refer to a person you are dating, married to, etc. It is generally used as a noun.

Self Ship is generally used as a verb. It means doing shipping related things with your F/O. (posting, creating fanart, etc) “I self ship with my F/O.”

Having a crush on a fictional character is a state of being which requires no action. A person with a crush on a fictional character may choose to call that relationship an “F/O” and proceed to engage in the action of self shipping.