Ex-Olympic sprinter (turned convicted killer) Oscar Pistorius was involved in a prison fight in the South African penitentiary where he's locked up ... officials confirm.
The Blade Runner -- who's serving a 13-year prison term for the 2013 shooting death of girlfriend Reeva Steenkamp -- got into the scrap on Dec. 6.
Prison officials say Pistorius and another inmate fought "over the use of a public phone in the special care unit."
Pistorius only suffered a bruise. No word on the other inmate.
No so tough without your gun, are ya?