If the Yankees and Orioles are tied at the end of the season for the division, there will be a one-game tiebreaker to determine the AL East champion. The winner of the season series – which will be the Orioles if they win Sunday – would host that game. If the season series is tied, then the host city is determined by the team with the better divisional record.
The Orioles currently have a four-game lead over the Yankees in games played within the division, but obviously that's not set yet. But the Orioles can guarantee home-field advantage in a one-game divisional playoff with the Yankees if they win Sunday.