You do have OhGodATool in HiveOS, it is just called wolfamdctrl.

There is no reason power consumption between Windows and Linux to be different (except for differences in the code being executed by the GPU because of different miner and/or OpenCL compilation). I wouldn't expect them to be too different if you are using the same miner in both.

Have you enabled "aggressive undervolting" option in the OC profile?

You have to make sure that the GPUs are honoring the OC profile. Use tools like "sensors" to check GPU core voltage (apt-get update && apt-get install lm-sensors)