'Ox' mainsprings are made in a few types and each type is supposed to fit a number of different guns, so the sizes are an approximation.
They DON'T have a very good reputation as they can be very harsh and the sharp edges of the square section wire can shave metal inside the gun.
Also, although this might sound strange, the most powerful spring does not necessarily make the gun more powerful, and can sometimes make it less powerful - and certainly less accurate because of the recoil.
If you decide to keep the Ox in your BSA do check the power as you may need to clip some coils if it has gone over the limit.