It's been confirmed by some media outlets, but I haven't seen anything from OU officially announcing his hiring.
I didn't know much about Simmons, other than he was a coach under Leach at Tech (with Riley) and Leach wanted him at Washington State (which is where he will be coming from). I looked up some information and came across a story on a Wazzu site. If you want to read it, here's the link. This part stood out most to me:
Losing Simmons would be a major blow for WSU. Leach has done a fine job of filling assistant coach spots as they've come open -- with WSU improving in many of the instances. Doing so if Simmons does depart will be a big challenge. He's a tremendous recruiter in Southern California, an area WSU hits hard, and a damn fine wide receivers coach.
So, he appears to be a really good position coach (and given how well the receivers have done at Tech and WSU under Leach, that's not surprising). And perhaps more importantly, he's a "tremendous" recruiter. Just what we need!