The 18-year-old will spend the season in the Netherlands after United took advantage of the later closing time for Eredivisie clubs to get the deal over the line, with a medical completing the formalities.
Willem II, newly promoted back to the Dutch top flight, had made an offer for the forward earlier in the week, seeing off separate interest from Feyenoord that had cooled without progressing into a concrete approach.
Obi made seven Premier League appearances and one in the FA Cup during the 2024-25 season under then-manager Ruben Amorim, but did not feature for United's first team at all last season, instead playing exclusively for the Under-18s and Under-21s. He scored 20 goals in 37 appearances for those age groups, including a run to the FA Youth Cup final in which United lost 2-1 to Manchester City, but had been left out of the club's opening Premier League 2 victories over Ipswich Town and Leicester City this term.
A Denmark Under-21 international, Obi joined United from Arsenal's academy and had been considered one of the more promising young forwards in the club's youth system.
With regular senior football clearly the priority for his continued development, a loan to a club fighting to establish itself back in the Eredivisie offers a route to first-team minutes that would have been difficult to guarantee at Old Trafford this season.
The move reflects a broader trend of Premier League academies sending teenage prospects to the Netherlands for senior experience, with the Eredivisie's reputation for developing young talent making it an increasingly common destination for English clubs looking to accelerate a player's progress away from the pressures of top-flight football.
Born in Copenhagen to Nigerian parents, Obi moved to England as a young teenager and joined Arsenal's academy before switching to Manchester United, where he quickly built a reputation as one of the standout young forwards in the club's youth ranks. Academy staff have previously praised his hunger for game time, with youth coach Darren Fletcher recalling how Obi had asked to play in a fixture at short notice rather than accept a weekend off.
Whether he can translate that appetite into senior goals at Willem II, who are expected to be among the contenders for relegation this season, will go some way to determining his path back into United's first-team picture.