Yes, you can not push BCLK freely on OS, you need to boot higher BCLK to get higher clock, if you boot with low BCLK you can't push it further as far as i know. So try to boot at 124bclk or more to get higher limit BCLK OC on OS before freeze.
@Nik What CPU Voltage did you feed them? Non K OC need higher CPU Voltage for boot because Core Clock & Cache Clock Ratio 1:1, and to get higher frequency you need to boot at high clock & bclk with enough CPU Voltage. So unless you have really good CPU, you may need to raise your CPU Voltage further if you dont.