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 the western of the long arms of the T-shaped hospital building (shown in grey and marked 1 in the map above). There are doors on both sides of the OR. On one side the door leads to the Pre-Op Ward (number 1 in the map above) where casualties were prepared for the O.R.
As for the other door, in Cementing Relationships, it can be seen that this leads to a small anteroom (probably a rest area) and through another door to the lab. Beyond this, or through another exit from the anteroom is probably 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 and the pharmacy. The short arm of the T-shape was the post-op ward where the patients were brought after surgery to rest and complete their recovery. The junction between the arms where the camp office and commander's office was located.
In "Bottoms Up" (Season 9), patients who have been operated on are shown being carried out of the operating room through the door to pre-op. New patients are brought in through the same door. This suggests that there is no direct indoor connection between the OR and post-op, or perhaps that going out through the other way required the stretcher bearers to weave through the lab or scrub room and then the camp office, which in fine weather was probably more inconvenient.
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.
the O.R. as seen in "O.R."(Season 2)
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.