The operating room, more commonly referred to as the "O.R." was the heart of the MASH. It was where the wounded were operated on and given life-saving treatment. Except for certain bottle shows, almost every episode had a scene in the O.R., as if to remind the viewers what the series was all about. At least two episodes, "O.R."(Season 2) and "Deluge" (Season 4) were set almost entirely in the O.R.

The O.R. in the 4077th was located in one of the long arms of the T-shaped hospital building. On one side was the pre-op ward where casualties were prepared for the O.R. and also a small rest area. On the other side was the scrub room where the O.R. personnel scrubbed and got ready for surgery. Nearby (although the exact locations can be difficult to determine) were such support facilities such as the X-ray room, the lab and the pharmacy. The other long arm of the T-shape was the post-op ward where the patients were brought after surgery to rest and complete their recovery. The short arm of the T was where the camp office and commander's office was located.

The core feature of the O.R. was the four operating tables. In the early part of the Korean War when the fighting shifted rapidly and the MASHes were constantly on the move, the patients were typically operated simply on litters placed on trestles or saw horses. However the 4077th as depicted in the M*A*S*H TV series represented a time in the Kporean War when the fighting had settled into a stalemate and the MASHes were fairly stationary. Under these circumstances the layout and equipment of the MASHes adapted and became more permanent in nature. There are contemporary photographs of MASH O.R.s of the Korean War showing equipment and layouts very similar to that depicted in the TV series.

Around the four operating tables there would be equipment on wheeled trolleys and also cabinets of medical supplies. Above the tables, powerful lights provided illumination. The floor of the O.R. was originally wooden planks but in "Cementing Relationships" (Season 9), the MASH personnel got together and laid their own cement floor because the wooden floor was difficult to clean and had become a source of infection.

Operating room-OR

the O.R. as seen in "O.R."(Season 2)

OR fridge-bottoms up

Seen in the Season 9 episode "Bottoms Up" was a fridge next to the cabinets of medical supplies. Here Klinger is removing a bottle of blood from the fridge.