Either **_or_** an abstraction related to both, depending on context.

To be clear, **"_OP_"** may refer to:

  *  the **O**riginal **P**ost; or

  *  the **O**riginal **P**oster; or

  *  the **O**riginal **P**ost _or_ the **O**riginal **P**oster, ambiguously;

  *  the **O**riginal **P**ost _and_ the **O**riginal **P**oster, 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.