A product of A.C. Milan's youth system, Osuji was loaned out to Varese for the 2009–10 season. He made his official debut for the club on 23 August, in a match against Perugia. On 22 November, he also scored his first goal, in a home game against Foligno. On 7 July 2010, Varese announced to have signed the Nigerian on a co-ownership deal, for €10,000.
In summer 2011 Padova bought Varese's half for €310,000.