The English abbreviation OP for the term Original Poster is widely used over the internet. Do the abbreviation and/or the term have widely used equivalents in Spanish?
I don't think there really is an accurate translation.
Of course you could literally translate Original Poster in various different ways, the point of OP is to make a very short abbreviation that is very easily recognizable.
So, given that there is no universal set for Spanish, any literal translation you choose will be ok but you won't be able to shorten it.
If it was me, if the site has a relatively internet active community, I would use OP, and explain if it is not clear.
I would simply use OP and be done with it. Thanks to the Academies in charge of its standardization, the official Spanish language often evolves more slowly than the needs of its speakers, so quite often you will end up having to choose between deviating from standard Spanish and not being able to say what you want to say.