I think in 286 systems there isn't really a standard method for determining the ISA bus frequency. Obviously it's better to have one of the later 286 boards where you could adjust the CPU divider in the BIOS.
I have heard that the ISA bus in the original IBM AT runs at 8MHz regardless of the speed of the CPU, but I am not so sure about that. If somebody has a bustest utility I could test this theory. What I do know for certain is that there exist badly designed 10 and 12MHz AT clone motherboards that run the ISA at the same speed as the CPU. 10MHz is easier to pull off on 8-bit cards, but 16-bit cards like SCSI and soundcards can have problems at 10MHz. 12MHz is really asking for trouble.
Finally, I don't know much about what the OSC signal is used for, but what I do know is that it not necessarily used for determining the operating frequency of the bus. Many ISA cards do not even use the OSC signal. But, I think I have seen some 486 motherboards that are capable of setting the ISA bus according to OSC. It's definetly not mandatory.