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If Midnight Oil has no hour counters on it, its first triggered ability will continue to trigger and have you draw an additional card, even though you can't remove any counters from it.

If Midnight Oil has only one hour counter on it, its first triggered ability will remove that one counter.

If multiple effects try to set your maximum hand size to a certain number or state that you have no maximum hand size, the one created latest is the one that applies.

If you discard a card during your cleanup step while you control Midnight Oil, its last ability will trigger, and players will get priority in your cleanup step. You'll have another cleanup step after that one before continuing to the next player's turn.

Your maximum hand size is only checked during the cleanup step of your turn. At other times, you may have more cards in hand than your maximum hand size.