Recently, my knee hurts after running. I used to run 5-6km twice a week. What are the chances of it being osteoarthritis?

Could be. It is most likely osteoarthritis but it could also be a knee injury. See your doctor for a physical examination if it doesn't improve or get worst.
Little more info. It would be unusual to have osteoarthritis at a young age so your age is important. It still could be an overuse syndrome even if it is only couple times a week, it might be worth taking a couple weeks off or decrease the distance you run. If there is swelling, warmth, or your pain is no longer associated only with running you will definitely need to see a physician..