The StarTrek.com website maintains an office listing for many main characters.
Jean-Luc Picard (Captain, USS Enterprise)
Office: Enterprise: Deck 1 Ready Room, adjoining Main Bridge
Geordi la Forge (Chief Engineer)
Office: Enterprise: adjacent to Warp Core, Deck 36 Main Engineering
Beverly Crusher (Chief Medical Office)
Office: Formerly, Enterprise-D: Deck 12/Room 1629
Guinan (Civilian Lounge Hostess)
Office: U.S.S. Enterprise: Adjacent to Ten-Forward lounge, Deck 10
Deanna Troi (Ship's Counselor)
Office: Formerly, Enterprise Deck 8.
Kathyrn Janeway (Captain, USS Voyager)
Office: (2371-2378) Voyager, Deck 1 Ready Room adjoining Bridge
The Doctor (Chief Medical Officer)
Office: (through 2378) Adjoining Sickbay on Deck 5, U.S.S. Voyager
Chakotay (XO, Voyager) has an office aboard the Voyager. It appears to be on Deck 2.
Tuvok (Chief of Security, Voyager) has an office on Deck 6, proximate to the brig.
B'Elanna Torres (Chief Engineer, Voyager) has an office that's directly adjacent to main engineering.