What is OSM?
Definition of OSM in Construction Outside measurement This is the exterior measurement of the entire window or door unit. This is an important dimension to ensure that the window or door unit will fit into the rough opening ( RO ) or the masonry opening ( MO ). The outside measurement is the overall measurement of everything that is included within the door or window unit. Again, this is an important measurement and must be carefully evaluated to ensure that the windows and the doors will fit into the openings within the exterior walls of the structure.