The Main Entrance
As far as I know, the main entrance to the office suite is seen only in the first season. Here are some samples.

Picture 1 Episode 1. The door has curious sidelights and is on a flat wall.

Picture 2 Episode 25. Out side of teh door is a hallway.

Picture 3 Episode 31. Still the odd sidelights but now the door is on a wall segment that prottrudes into the room.

[The Office][Perry Mason TV Series]