O'Shaughnessy joined Championship club Brentford on trial in July 2014 and made a 30-minute cameo in a 4–0 pre-season friendly defeat to Osasuna on 29 July. On 1 August, it was announced that O'Shaughnessy had signed a two-year professional contract. He was an unused substitute during Brentford's two League Cup ties in August 2014, but instead spent the 2014–15 season in the Development Squad, in which he made 23 appearances and scoring one goal. O'Shaughnessy began the 2015–16 season in the Development Squad and made 13 appearances and scored one goal before leaving on loan for the rest of the season in late January 2016. He was released in May 2016, having failed to make a first team appearance during two seasons at Griffin Park.
On 22 September 2015, O'Shaughnessy joined National League club Braintree Town on a one-month loan. His only appearance for the club came later that day as a half time substitute for Sam Habergham during a 2–1 win over Woking.
On 28 January 2016, O'Shaughnessy joined Danish Superliga club Midtjylland on loan for the rest of the 2015–16 season. He won his maiden first team call-up for a league match versus Copenhagen on 3 March and remained an unused substitute during the 1–0 defeat. After four further substitute appearances, O'Shaughnessy made his debut in the final match of the season versus Nordsjælland and replaced Kian Hansen after 59 minutes of the 4–1 victory.
Cheltenham Town
On 12 July 2016, O'Shaughnessy joined League Two newcomers Cheltenham Town on a one-year contract. During a 2016–17 season in which the Robins narrowly avoided being relegated straight back into non-league football, he made 36 appearances and scored four goals. O'Shaughnessy signed a new one-year contract in June 2017, but he was down the defensive pecking order during the first half of the 2017–18 season and he departed the club on 2 January2018. O'Shaughnessy made 49 appearances and scored four goals during 18 months at Whaddon Road.
Return to HJK
On 2 January 2018, O'Shaughnessy returned to Finland to rejoin reigning Veikkausliiga champions HJK on a two-year contract, with the option for a further year. He made his debut appearance for the team on 17 February 2018 in a Cup match against KTP. Despite missing the final three months of the 2018 season with a knee injury, O'Shaughnessy made 24 appearances, scored three goals and in his absence, HJK were crowned Veikkausliiga champions. O'Shaughnessy was a near ever-present during the 2019 season and finished the campaign with 31 appearances and three goals. During a period while he was conducting his national service, O'Shaughnessy signed a new two-year contract in October 2019.
International career
O'Shaughnessy represented Finland at all age-groups from U15 up to U21 level. He won his maiden call into the senior team for a friendly versus Estonia on 9 June 2015, but remained an unused substitute during the 2–0 defeat. O'Shaughnessy made his full international debut as a late substitute for Ville Jalasto in a 3–0 friendly defeat to Sweden in Abu Dhabi on 10 January 2016. He made his first start three days later versus Iceland and played the full 90 minutes of a 1–0 defeat. Two years after his first call-up to the senior team, O'Shaughnessy won his third cap as a half time substitute in a 2–1 friendly win over Jordan on 11 January 2018. He remains eligible to play for the Republic of Ireland through his Irish father.
Personal life
O'Shaughnessy was born in Riihimäki, Finland, to an Irish father and a Finnish mother and holds dual Finnish-Irish citizenship. O'Shaughnessy's older brother Patrick is also a professional footballer.
