I think I talked about it in this blog entry, but this time it was for the slang meaning.
It is used when a person has a relationship with a parent and that person's child.
The first "oyako donburi" manga that comes to mind is Yukari Ichijou's "Tadashii Ren'ai no Susume."
It translates to "Recommendations to a Correct Romance." or "The Correct Way to Love." It actually gives the wrong way to go about relationships, because the main character Hiroaki is going out with Miho, but also has sex with her mother Reiko (without knowing at first) as part of his escort job.
It's bad because he continues the relationships with both Miho and Reiko.
The yaoi I worked on doesn't actually have oyako donburi. Just to clarify.