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At the beginning of your end step, if creatures you control have total power 8 or greater, draw a card.
Owlbear Shepherd's ability has an intervening if clause, which means that it checks to see if creatures you control have total power 8 or greater both as it would trigger and as it resolves. If enough creatures have been removed or their total power is reduced to below 8 by the time the ability tries to resolve, you will not draw a card.
