It's pronounced roughly as "O-hem-ma".
Breakdown:
* Ɔ: This is the "O" sound with a bit of a "w" before it.
* hem: This is the "hem" sound, similar to the English word "hem".
* maa: This is the "ma" sound, similar to the English word "ma".
Let me know if you'd like to know other Twi words!