If I understand, you just want to add a Bisley hammer and trigger to an OM BH with the standard OM grip frame.
1. You'll need a Bisley hammer made for the OM by Power Customs:
http://powercustom.com/store/index.php?main_page=product_info&cPath=12&products_id=1118
But since it's made to be used with a Bisley grip frame, it needs the hump ground down to fit the standard plow-handle grip frame, by scribing the hump on your factory hammer onto the Bisley. Then gringing off and polishing.
2. You'll need a Clements OM adapted Bisley trigger. This wide trigger will require the OM trigger slot widened unless you use an OM Super BH grip frame or Old Army steel grip frame, blue or stainless steel. The shelf on the back of the trigger will have to be ground off to work with the OM trigger return spring.
___________________________Clements adapter triggers are the center two_________________________
OM trigger__________________standard____&____Bisley w/over travel stop__________________NM Bisley
They are $65. http://www.clementscustomguns.com/