The Ligue 1 club, owned by the same investment group as Chelsea, will hand Sevilla a 10 per cent cut of any future sale as part of the structure.
The 23-year-old, whose contract had been running down without a renewal agreed, had been the subject of interest from Fiorentina earlier in the summer, but it is Strasbourg who have won the race, tempting him with a significant pay rise and a deal running until 30 June 2031.
Oso made only 27 appearances for Sevilla's first team, scoring three goals and providing four assists, but had established himself as a starter in both of the club's opening two LaLiga matches this season, contributing decisively to wins over Rayo Vallecano and Athletic Club.
His departure leaves García Plaza short on the left flank at a time when Switzerland international Ruben Vargas is also being frozen out of plans while a sale is negotiated, forcing Sevilla's sporting department into the market for reinforcements with barely any of the window remaining.
The sale has created visible friction between García Plaza and the club's hierarchy, led by sporting director José Ignacio Navarro. The manager described the news as "a blow" and admitted feeling "in shock" in comments made in the days before Oso's exit was confirmed, with the club insisting the move was necessary to generate the salary cap space needed to complete further incoming transfers.
Sevilla's financial position, burdened by significant existing debt, meant a fee close to the club's original asking price - after an earlier €7m offer was rejected in May - was too significant to turn down.
Oso came through Sevilla's academy after arriving from Málaga's youth system in 2022, making his first-team debut in the Copa del Rey in December 2023 before establishing himself under Matías Almeyda last season, when he played a key role in the club's survival battle.
Born in Torrevieja to a Spanish father and Argentine mother, he holds dual nationality and had reportedly appeared on the radar of Argentina coach Lionel Scaloni, adding a further layer of intrigue to his sudden departure from Nervión.