Oscar Pistorius -- Arrested in 2009 for Assaulting a Woman

Olympian Oscar Pistorius -- who now stands accused of murdering his girlfriend -- was arrested in 2009 for allegedly assaulting a female friend ... TMZ has learned.

Boschkop police in South Africa tell TMZ ... Pistorius spent 24 hours in jail following his 2009 arrest ... before the alleged victim decided to drop the charges.

Pistorius is currently being held at a nearby Pretoria Prison.

According to Boschkop police, Oscar and his girlfriend Reeva NEVER had any prior incidents ... at least where cops were called.