Everybody here seems to have some difficulty understanding how unions work.
The union represents the workers who are members. Period.
The collective bargaining agreement (CBA), aka contract is nothing but an agreement between the union employees and management.
Both sides can, and do, violate that agreement whether it be when certain classes of workers do work outside their class, or work OT outside the agreement, or if management directs workers to do things that are not in accordance with the agreement.
There is a process by which both sides can pursue corrective action in cases where the contract is violated. The vast majority of those would be through grievances filed by the union employees. Then management and the union will take that through the process to come to some agreement as to whether it was a violation, and how the affected union employees should be compensated (most cases).
Management can, contrary to opinions stated earlier, terminate any employee, union or not, for whatever reason they desire. The union can file the appropriate processes to fight that, and if that employee was terminated in violation of the agreement, the company will suffer. Not often will the NLRB or a court force them to take back an employee they believe to have been terminated appropriately, though I have seen that before. More likely, they'll reach an agreement on how to compensate the terminated employee, and possibly, fine the company for violating the contract, and therefore, labor laws.
If Kevin Ollie is that employee, the best he can hope for is his entire buyout. That's it. They can't force UConn to take him back (especially in this case given his actual job performance over the last two seasons) or to prevent them from hiring someone else.