Can i run an 8km if i have an osteoarthritis-like condition on my right ankle?

Varies: It varies somewhat. What you would want to do is gradually increase your distance running to see the level of activity that is well tolerated, which may or may not be an 8k run. Some people will also be able to do more in a run if they do a "run-walk" as well rather than a continuous walk. This was popularized by Chris Galloway as a way for more people to run without injury.