Other People's Rubber. A term autocrossers use to describe tire rubber that has worn off racing tires, usually building up in piles off the proper racing line. A hindrance to tire grip, OPR can be dragged on course by competitors during their course walks causing Porsches Caymans to be reduced to stock Miata speeds.
I would have gone faster on that run, but my tires lost grip in the slalom because somebody spun before me and dragged some OPR on course.
I had a 2.5" wide continuous line of OPR on my front tires, removing my effective contact patch by over 60mm, or making my tires effectively 205 instead of 285.