On 29 September 2005, at the age of he made his debut with the first team in the UEFA Cup, playing against Aris. He thus became the youngest footballer to debut in an international competition for an Italian team. Okaka scored his first goal for Roma in a Coppa Italia match against Napoli on 8 December, with a powerful run from midfield and a precise shot to the left of the goalkeeper, making him the youngest goalscorer in the competition at the age of . He made his Serie A debut with the club later that month, on 18 December, at the age of 16 years and 125 days, in a 1–1 away draw against Sampdoria.
