Oper played as a forward for Lelle, Flora, Tervis Pärnu, AaB, Torpedo Moscow, Roda JC, Shanghai Shenhua, ADO Den Haag, AEK Larnaca, Nea Salamina and the Estonia national team.

[4] On 10 July 2003, Oper signed a two-year contract with Russian Premier League club Torpedo Moscow.

[5] Often inconsistent in Russia, scoring 8 goals in 53 appearances, Oper was placed on the transfer list after he suffered an injury to his right foot.

[6][4] On 31 August 2005, Oper signed a one-year contract with Dutch Eredivisie club Roda JC for an undisclosed fee.

[10] On 19 July 2009, Oper signed a half-year contract with Chinese Super League club Shanghai Shenhua.

[12] After an unsuccessful spell in China, Oper returned to the Netherlands and on 21 January 2010, he signed a half-year contract with an option for another year with ADO Den Haag.

[15] On 9 September 2010, Oper signed a one-year contract with Cypriot First Division club AEK Larnaca.

In 2021, he notably scored the winning goal in an Estonian Cup match against top flight club Pärnu Vaprus.

[19] Oper made his international debut for the Estonia national football team on 19 May 1995, in a 0–2 1995 Baltic Cup defeat against Latvia.