Both are correct depending on context. "at" usually answers the question "where" "in" usually answers the question "inside" or "outside" / "here" or "away" (casual conversation with John's friend) Where is John? John is at work. (phone call) May I speak to John? No, John is out of the office. (= He is away) / Yes, John is in the office. (= He is here.) There are some general rules (patterns) for "at," "in," and "on" but there are many exceptions. "at" is the most common.

I usually teach that *at* as a preposition of location is our go-to preposition. while at and in are both correct, we dont really need to know if you are inside the apartment or outside. If we were both at your office, in the office might be more appropriate so I don't spend time walking around outside looking for you.

If you're referring to your place of work, we would usually say "I am at the office" because "the office" is referring to a more general location. In other words, you're simply saying, "I'm at the place where I work" - this could be at your desk, in a conference room, in the WC, or outside smoking a cigarette. Sometimes, people have a room in their house that they designate the "office." If you are referring to a specific *room*, you would say "I am in the office," just as you would say "I am in the kitchen" or "I am in the bathroom." You might also say this while at your work place if you are emphasizing your physical location in a specific room/space ("John, I can't talk now, I'm in the office" emphasizes you are *in* the office building, implying that people around you are working and don't want to listen to your phone conversation!).

depends on the context

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