after applying thermal paste and mounting my CPU fan, I discovered the power cord for it is too short to reach the CPU_FAN pins. However, it can reach the CPU_FAN_OPT pins, is it ok that I can plug into that one instead? Or does it have to be in CPU_FAN?
It will work on the CPU_FAN_OPT header as well, there's no doubt about it. If it is a header supporting PWM fans, even better, otherwise it will just function like a normal case fan. The motherboard may require you to ignore CPU_FAN speeds in the BIOS though, or it will keep bugging you every time you switch on the PC, but that's easy to do and takes 5 minutes to complete.