Well, if you ask Oklahoma State, they say they are actually suing Texas "offensive line coach" Joe Wickline.
Here's the short version. Wickline's contract with Oklahoma State (where he was Mike Gundy's offensive line coach) called for specific liquidated damages in the event he broke his contract to leave for any position other than a head coaching position or an offensive coordinator role ("with play calling duties") at an FBS program or as an assistant on an NFL staff.
Wickline joined Charlie Strong's staff as offensive line coach and was given the offensive coordinator title as well to help him avoid having to pay the required damages to Oklahoma State. I say "given the title" because it was clear to everyone in the profession that Shawn Watson ("assistant head coach for offense") would actually serve in what is traditionally considered the offensive coordinator's role. Watson takes the lead on the game planning and calls the plays from the box.
Well, now Oklahoma State wants their money, to the tune of $593,000 and change. Let that sink in. For those that are into reading lawsuits, here is Oklahoma State's petition.
Note that Wickline has also countersued Oklahoma State now too. Wickline is largely seeking full release from his contract at Oklahoma State. The Austin American-Statesman has a good writeup on that end.
No one knows where this goes from here; but my guess is the parties reach a settlement in which Wickline sends some amount of money back to Oklahoma State. He won't be happy about it though. Pretty unclear if Texas would help him with any of that. Wickline currently makes $575,000 per year at Texas (assistant head coach for offense Shawn Watson makes $650,000...see Oklahoma State's point?). Let's just say, things just got real in Austin.
You ask, how clear is the clause in Wickline's contract about him having to call the plays, well here it is:
(note: I highlighted the phrase in yellow)