"Declaration of War" (Japanese: 宣戦布告 せんせんふこく , Sensen Fukoku) is the twenty-fifth episode of Haikyū!!

What episode is Oikawa in Season 4?

Season 4, Episode 23 brings back fan-favorite character Toru Oikawa for a brief but thematically significant cameo.

What happened to Oikawa in the end?

After his final year, Oikawa goes on to join the Club Atlético San Juan in Argentina, where he became a citizen, and even played on the Argentian National Team for the 2021 Olympics against some familiar faces (some of whom he'd kept in touch with).