Story ideas first, mostly. I know I'm a writer, but I used to draw when I was younger and have several OC's. But, until now, there are only two OC's (siblings) that I'm trying to incorporate to a plot (I haven't written the story yet, but they're in the character list). And that project is the only one where I'm trying to design my character first before plotting the story.
Some of my OC's were used solely for OC roleplay. They do have backstory, but that's it. I just don't feel comfortable turning my OC's into characters in my stories, especially reusing them in several stories. When I'm creating a story, I will create new characters. So, story comes first, character fits in the story second.