What it means
や否や indicates that one event happens immediately after another, with virtually no interval between them. It attaches directly to the dictionary form of a verb and presents the second event as occurring the moment the first event begins or is completed. The construction is highly formal and is especially common in written narrative, reports, and descriptions of rapid sequences.
Usage note
や否や is a formal written construction and is not normally used in casual conversation. It takes a verb in dictionary form, not the ます-stem, and the following event is generally presented as an actual occurrence rather than an intention, request, or command.
Example sentences
警報が鳴るや否や、職員たちは来館者を安全な場所へ誘導した。
The moment the alarm sounded, the staff guided the visitors to a safe place.