Born Erbach im Odenwald, Hessen, West Germany, Nikolov grew up in Sandbach in Hessen, starting playing football with local SG Sandbach. At nearly 17 he joined Eintracht Frankfurt's youth system, going on to remain with the club until his retirement.
Nikolov started his senior career as backup, first to Uli Stein and then Andreas Köpke. After the latter left the team in June 1996 following the team's relegation from the Bundesliga he became first-choice, losing that status in early 2002 to newly-signed Dirk Heinen; he made his debut in the top division on 9 September 1995, playing the full 90 minutes in a 3–1 home win against 1. FC Kaiserslautern.
In the following years Nikolov was relatively played in every season, being an undisputed starter in five of those but only appearing once in the 2004–05 campaign as the Eagles achieved promotion from the second division, playing second-fiddle to Markus Pröll. In February 2007, after Pröll relapsed from his rib injury, he was reinstated by manager Friedhelm Funkel, contributing with 20 games to the team's permanence in the top level.
In January 2010, it was announced that the 35-year-old Nikolov would not renew his contract with Eintracht that was due to expire in the summer, but he eventually signed a new deal until 2012, and later until 2013. In June 2013, Eintracht revealed that the goalkeeper's contract would be cancelled, effective immediately to allow Nikolov to fulfill his lifelong dream to play in the United States. Thus, after 22 years of fidelity to Frankfurt, Nikolov left the club as Germany's most loyal Bundesliga player.
On 19 June 2013, Nikolov signed with the Philadelphia Union of Major League Soccer. He made his first start as a Union player on 30 July 2013 in a friendly against Stoke City playing 45 minutes. His main responsibility though was to guide the younger Goalkeepers in the club to develop their performances.
Nikolov signed with Fort Lauderdale Strikers on February 19, 2014 on a contract for the nine-game Spring season. Nikolov started and played eight matches in goal for the Strikers, posting two clean sheets. The team ended up as runners-ups.Director of Coaching at JFC Frankfurt
Project Director and Coach at the Eintracht Frankfurt Football Academy
Assistant Coach for the under 19 German National Team.
Nikolov also served as Assistant Coach for the Macedonian National Team.
Currently, Oka Nikolov serves as the Assistant Coach for the Philadelphia Union (MLS).
Nikolov chose to represent the country of his ancestors, going on to win five caps for Macedonia in four years. Since he also possessed German citizenship he couldn't acquire a Macedonian passport and therefore was unable to play in official games under FIFA. His 5 appearances were only in friendlies.DFB-Pokal: Runner-up 2005–06