A gay male/female who considers himself/herself dedicated to you, like a manservant with a crush on you. There may be but often is not a sexual context to it. Often it's because you're the one to whom he/she came out first, but there can be other reasons. This is a very caring, kind and maybe even loving person if you're lucky, and he/she may be vulnerable, too. Don't abuse the privilege.
I love my OPF! He's always there to bail my ass out of jail after (fill in the blank).
I'm in between girlfriends so my OPF is filling in for a few days. He's good-looking, rich, and sucks like a Hoover, glad he still wants me. Life's damn good!