IDK if anyone has said this already, but many servers have auto-login on their authorization plugins. To test it, login to the server regularly and log out. Then log back in. If it says something like "Your session has been resumed" then your good. Then, you can wait for an admin to log in. After that, wait for him to log out. Immediately after he logs out, you log out and then log in as him. If you can join in time, you will be resumed in his session, and that is when you OP yourself.