Oathsworn Knight — ruling 1

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 — ruling 2

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.

Oathsworn Knight — ruling 3

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.

Oathsworn Knight — ruling 4

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

Oathsworn Knight : règles, fonctionnement et interactions (MTG)

Oathsworn Knight is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This creature enters with four +1/+1 counters on it. This creature attacks each combat if able. If damage would be dealt to this creature while it has a +1/+1 counter on it, prevent that damage and remove a +1/+1 counter from it.

How it works : Oathsworn Knight

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2019· Wizards of the Coast
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.
No other card is named in this ruling.
#2October 4, 2019· Wizards of the Coast
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.
No other card is named in this ruling.
#3October 4, 2019· Wizards of the Coast
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.
No other card is named in this ruling.
#4October 4, 2019· Wizards of the Coast
If damage that can't be prevented is dealt to Oathsworn Knight, one counter is still removed from it.
No other card is named in this ruling.