Either or an abstraction related to both, depending on context.
To be clear, "OP" may refer to:
the Original Post; or
the Original Poster; or
the Original Post or the Original Poster, ambiguously;
the Original Post and the Original Poster, abstractly.
When it's ambiguous, the writer may be thinking of the post or the poster. They may fail to specify which for a variety of reasons, ranging from believing that the definition normally refers to the one that they intend to simply not caring to specify.
However, others mean "OP" abstractly as "the original source of the content". They're not actually referring to either the post or the poster specifically, so we can't say that their usage of "OP" maps to one over the other.
The distinction between this abstraction and ambiguity is the presence-or-absence of referential transparency. For example, LOL might be said to mean "Laugh Out Loud", but that's not actually what anyone means by it. It's a distinct concept; rewriting "LOL" as "Laugh Out Loud" would change the meaning of the text.