Thanks to the hint by leehoon0556 via tweet to dakaya007, stating that Onew went back after his filming was done (trans by jujugal@soompi).
For more info on the drama, go here: Doctor Champ. This is already the last episode, and Super Junior's Shindong also happens to be one of the regulars.
Edited to add a bit more info on the drama.
And to think Minho's having his drama debut first. Oh Dubu, full of surprises.