OC depends on the processor. "Locked" processor most of the time refers to it's multiplier only, it's locked so it can't be raised more than stock. Some of them can be OCed even with locked multiplier.
For example, AMD Athlon II and Phenom II processors without BE (Black Edition) at the end of designation can be overclocked a lot using FSB only. In some of them that are x2 or x3 even another core or two can be unlocked. Phenom II x2 can be unlocked to x4 and OCed a lot at same time.
AMD FXxxxx processors all have unlocked multipliers, even ones that are not Black Edition which only means that they are C0 stepping and therefore higher binned which in turn means they can have better overclock.