The pair may look for moves to the Premiership having missed the Celtic League's registration deadline of January 31.
The news that former Harlequins player Llewellyn, 35 this month, is being let to go comes just days before the start of the championship.
"The region have reached agreement with Gareth Llewellyn to allow him to explore new career opportunities," said Ospreys joint managing director Roger Blyth.
"I think given his commitment to Wales, the Ospreys and before that Neath, it is only right and proper that we give him every assistance as he decides what course he now wants his career to take.
"There will be no hasty decisions and the important thing is that Gareth is given every respect, the room and the time to make the choices he now wants to make.
"We have also taken the difficult decision to release Gavin Thomas onto the transfer list which, despite a good win on Saturday, signals the beginning of tough choices we are now taking in a process which amounts to a redefining of the squad.
"This is a programme that will continue until the end of the season and we expect to considerably modify and strengthen the squad in a process that will not just be about playing ability but about cultural fit, too."