Reddit glossary
What is OP on Reddit?
OP means original poster, the person who started the thread.
People use it to talk about whoever wrote the first post. You will see it in comments like "OP asked about budget, so here is a cheaper option".
It can also mean original post, depending on the sentence.
If you are promoting something
When you reply to a thread, answer what OP actually asked. A reply that ignores the question and pushes a product is the type of thing people downvote.