OP is a common term used in the battledome. It can either be an abbreviation for One Piece (with variations, such as OPverse), or it could mean Opening Post/Poster.
The OP is the first post in a thread, or the person who made that post. The OP is allowed to define the terms of the match, overruling the General OBD Assumptions, or adding extra stipulations (such as making the combatants' speed equal, in order to prevent speedblitzes). The OP can also change the conditions while the thread is in progress, but it should be noted that this privilege should not be abused. An example of abusing this power is to constantly redefine your terms to exclude every result you don't like. For example, when jplaya2023 made a threat claiming the Saiyans were the strongest race in fiction, he kept changing the rules to disqualify any races that were superior. Eventually, he had made the qualifications so narrow that all he had proved was that the Saiyans were the strongest race in fiction, excluding any race that wasn't exactly like the Saiyans.
However, the OP rule rarely leads to problems like this, and generally works well.