Formed atDeportivo Saprissa in Costa Rica, he moved to Europe in 2013 to play forClub Brugge in Belgium. He spent seven years in Spain'sLa Liga withEspanyol andLevante, making 111 appearances and scoring four goals.
On 26 January 2016, Duarte agreed a permanent transfer toRCD Espanyol in Spain. The three-year deal cost the club €1.15 million and included abuyout clause of €15 million.[4] He made his debut five days later, coming on at half time and scoring anown goal in a 6–0 loss atReal Madrid.[5] In his next game on 14 February, he scored his only goal in 57 games for the club fromBarcelona, opening a 2–1 loss atValencia CF.[6]
Duarte moved to fellow La Liga teamLevante UD on 1 August 2019, on a two-year deal,[7] though the signing was not registered for three more weeks due toUEFA Financial Fair Play Regulations.[8] His first season in the city ofValencia was marred by serious injuries; on 21 January 2020 he scored his first goal to equalise in a 3–1 loss atSevilla FC in the last 32 of theCopa del Rey.[9] He missed only three league games in2020–21, scoring in two minutes to open a 4–3 home win overReal Betis on 29 December.[10]
On 30 April 2021, Duarte added another year to his Levante contract by playing his 26th game of the season, againstRC Celta de Vigo.[11] He chose to let his deal expire in May 2022.[12]
In the team's opening match, he scored his first goal forLos Ticos in a 3–1 defeat ofUruguay.[15] In their second match, the team beatItaly, a 1–0 win that qualified Costa Rica for theknockout stage.[16] Costa Rica completed the group stage unbeaten, recording a second consecutive clean sheet in a 0–0 draw withEngland inBelo Horizonte.[17] On 29 June, Duarte wassent off for receiving twoyellow cards in Costa Rica'sround of 16 match againstGreece. The team advanced via apenalty shootout to the quarter-finals for the first time in their history, where they lost on penalties to the Netherlands.[18]
In May 2018 he was named in Costa Rica's 23-man squad for the2018 FIFA World Cup in Russia.[19]
^Oviedo, Steven; Brenes, Cristian (13 February 2016)."Óscar Duarte marcó su primer gol en España" [Óscar Duarte scored his first goal in Spain].La Nación (Costa Rica) (in Spanish). Retrieved31 August 2023.
^Sancho de Rosa, Luis (22 May 2022)."Duarte anuncia su marcha del Levante" [Duarte announces his exit from Levante].Diario AS (in Spanish). Retrieved31 August 2023.