Will a hollowtech shimano BB fit a bike which currently has an octalink bracket?
The bikes themselves - in this case, the frame - are pretty much standard on all the modern mountain bikes.

The Hollowtech and Octalink systems are different because you need to match the crankset to the bottom bracket, that is, you cannot use the BB from one system with the crankset from other system.

But any of the bottom brackets can be installed in the same type of frame (the "default"/ISO 68mm wide, 1.37in diameter, 24 TPI).

Hope it helps