If damage that can't be prevented is dealt to Oathsworn Knight, one counter is still removed from it.

If multiple sources would deal damage to Oathsworn Knight simultaneously (for example, multiple creatures blocking it), its last ability applies once. All of that damage is prevented, and only one counter is removed.

If Oathsworn Knight can't attack for any reason (such as being tapped or having come under that player's control that turn), then it doesn't attack. If there's a cost associated with having it attack, its controller isn't forced to pay that cost, so it doesn't have to attack in that case either.

Oathsworn Knight's last ability removes only one counter from it each time it would be dealt damage, not one counter per 1 damage prevented.