Ola Aina signed for Chelsea as an under-11 and made his youth team debut as a schoolboy in the 2012–13 season and went on to start in both legs of the semi-final and final of the FA Youth Cup. In his youth career, he has also represented Chelsea in the under-18s, under-19s, and under-21 levels. On 19 July 2014, Aina made his first-team debut in a pre-season friendly against AFC Wimbledon, after Todd Kane’s injury. Aina started the match and was substituted at half-time for Branislav Ivanović as Chelsea went on to win 3–2.

Before the 2015–16 season, he was included in the pre-season tour, playing three matches in the International Champions Cup. After impressing Chelsea manager José Mourinho, Aina was included in the first-team squad for the campaign. Although he spent the entire season training full-time with the first-team, he continued his role in the under-21s and under-19s sides. On 23 September 2015, Aina was included in the match-day squad against Walsall in the fourth round of the League Cup, although he was an unused substitute.

On 6 July 2016, he signed a new four-year contract, ending the rumours of him leaving the London side. After signing a new contract, Aina was included in the travelling squad to Austria and the United States. He went on to play in all six pre-season matches. On 23 August 2016, Aina was given his professional debut by manager Antonio Conte in a 3–2 victory over Bristol Rovers in the EFL Cup, being replaced by John Terry in the 77th minute. On 15 October 2016, he made his Premier League debut in a 3–0 victory over reigning champions Leicester City, replacing goalscorer Victor Moses in the 82nd minute.

On 11 July 2017, Chelsea announced that they had loaned Aina to Championship side Hull City. He made his debut on the opening day of the season, 5 August 2017, away to Aston Villa, in a 1–1 draw. He scored the only goal in a 1–0 defeat of Blackburn Rovers in the third-round of the FA Cup on 6 January 2018.

On 28 March 2017, Aina was pictured alongside Chuba Akpom of Arsenal after discussions with Nigeria Football Federation president, Amaju Pinnick. In May 2017, Aina pledged his international future to Nigeria and obtained a Nigerian passport in preparation to making the switch from England to Nigeria. In the same month, he was called up for the first time to play for Nigeria.

On 14 August 2018, Aina joined Italian club Torino on a season-long loan after signing a new three-year deal with Chelsea. He made his Serie A debut a few days later in Torino’s 0–1 opening day defeat at home to Roma, coming on as a substitute in place of fellow right-back Lorenzo De Silvestri who went off injured after just 25 minutes. Aina scored his first goal for Torino in a match against Udinese on 2 February 2019. Aina made 30 Serie A appearances in what was a strong season for Il Toro, whose seventh place finish secured a Europa League qualifying spot.

In May 2018 he was named in Nigeria’s preliminary 30 man squad for the 2018 World Cup in Russia. However, he did not make the final 23. He was later recalled into the squad and played a major role in the team’s qualification for the 2019 Africa Cup of Nations held in Egypt. He played the first match against tournament debutant Burundi, assisting the only goal of the match.

On 11 June 2019, Torino triggered Aina’s option to permanently transfer from Chelsea for a reported fee of £9 million. He was a regular again during his second season in Turin, playing 37 times in all competitions.

On 11 September 2020, Aina returned to the Premier League, signing for Fulham on an initial season-long loan with an option to make the deal permanent.