Hello, there a couple of differences usually despacho is for lawyer but also is common use for office of person that have a high rank in the company or a duty like directors. Other main different is when you work en una office mean you are saying more about the company itself and the kind of job. Other different is the kind of job. If you office is separate for the rest like a in a totally separate room you call despacho but if is in a common place is more common call it oficina. I hope this help to notice the differences.

Hi, In my experience, yes, they're both interchangeable as far as I can tell. My feeling is that certain offices are more often call "despachos" (abogados being the one that comes to mind). As a matter of fact, I have the feeling that's more often associated with lawyers, though I don't have any solid scientific evidence of this :) If you have some examples maybe I could tell you if they sound "odd" as despachos.