The office is the Horzine's research offices, which are currently overrun. It is a multi-story building that eventually leads to a room with an open sky.
The first floor normally has an item behind the front desk, as well as having a trader. Against light waves, it is possible to setup a defensive fire zone by welding the door to the back spiral staircase, and shooting enemies that arrive by the front. This tactic is risky against prolonged assaults, or when enemies start pounding on the door while you are trying to repel enemies.
The second floor has a cubicle farm, and two corridors one infront and on in the back. A common defense with multiple players is to guard the front stairwell (at both top and bottom), as the lower corridor allows more time before the enemies arrive. At most, there only needs to be two players at the lower portion of the stairwell.
The third floor is a set of personal offices and a presentation room. Normally, players guarding the front stairwell will weld the door to the nearby presentation room, and congregate here.
The roof has three entrance points, two of which can be welded shut. If working alone, welding one after fleeing from perusers will slow them down, but you still need to be prepared against enemies coming from the two other directions.