Dronjak was born in the Swedish town of Mölndal to a Serbian father and a Swedish mother. His first instrument was the recorder; he then played trombone for a few years until, at the age of fourteen, he settled on guitar. Soon after, he started his first band, called The Hippie Killers. In 1989 he started a death metal band called Desecrator, which was later renamed Ceremonial Oath. But just before they released their first album, he quit the band. The end of his career in Ceremonial Oath was the beginning of Hammerfall. At first, Oscar's main priority was the death metal band Crystal Age. Thus, Hammerfall was initially merely a side project, where he and some friends were just rehearsing some songs that he had written, one of them being "Steel Meets Steel". But soon, they recorded their first album, called Glory to the Brave. Oscar uses Jackson guitars, most notably the Jackson Randy Rhoads. In 2011 he signed a contract with ESP Guitars.