The correct preposition here is "at". English uses "at" to indicate presence at an event or gathering, such as a party, meeting, or function. The correct sentence reads, "I did not see you at the office party."
"In" is used for enclosed spaces or larger areas, like a room or a city, not for events, so "in the office party" is unnatural. "For" indicates purpose or duration and does not fit the sense of physical presence at an event. "On" is used with days, dates, or surfaces, such as "on Monday" or "on the table", not with events like a party.
Since "at" is the standard preposition for indicating attendance at a social or work event, option 3 is correct.