Most makes of clipper including Oster, Andis, Wahl have a latch which is
depressed to release the blades. The latch is commonly situated under the
hinge assembly and holds the blade with a simple hook at the tip which catches on the blades.
If the blades will pull off without depressing the latch, chances are the tip has broken off and will require replacement.
Undo the two screws securing the hinge into place and remove both parts, when fitting the new latch make sure the spring ends of the hinge locate through the holes on either side of the latch.