The OP is an anime's opening sequence: the title song and credit montage, usually about ninety seconds, that begins most episodes. It is numbered when a series changes it (OP 1, OP 2, and so on).
The OP is the series' front door, and productions treat it that way. It is cut to sell the show's world and energy in ninety seconds, often animated by top staff, refreshed when a story changes phase, and packed with imagery that only makes sense later. Fans learn to read an OP the way they read a poster: who stands together, who is missing, what is shown in silhouette.
The song is half the artefact. OP themes are released as singles, chart seriously, and become inseparable from the series in memory; a generation can be summoned by four bars. Series and singer promote each other in a media mix loop that has run for decades.
Skipping the OP is a modern streaming habit and a mild fandom controversy, because the sequence is also where credits live and where a series states its themes. This site's position is uncomplicated: the OP is part of the work, its contents are fair game for questions, and its song is a fact of the series like any other.
In use"When the new OP replaced the old one mid-run, the changed silhouettes told fans the story had turned."