A central defender, Osborne began his career with amateur club South Ruislip Rangers and spent a period with Queens Park Rangers, before trialling with youth system at Brentford at the age of 15, on the recommendation of Bobby Ross. Osborne embarked on a scholarship at the beginning of the 2004–05 season and progressed sufficiently to be named as a substitute in a youthful squad for a Football League Trophy first round match versus Milton Keynes Dons on 28 September 2004. Osborne remained unused during the 3–0 defeat and was named on the bench on two further occasions, before making his debut on the final day of the 2004–05 regular season versus Hull City. He played the full 90 minutes of the 2–1 comeback victory, but did not appear in the Bees' 2005 League One play-off campaign. Osborne signed a professional contract and made three appearances during the 2005–06 season. He spent the final two months of the season on loan at Conference South club Hayes.