Athletics pitcher Liam Hendriks explains how being counted out has been helpful for the team heading into the wild-card game against the Yankees. (0:26)

The Boston Red Sox are the World Series favorites, followed closely by the defending champion Houston Astros, but another American League squad is the biggest concern for one Las Vegas sportsbook as baseball's postseason begins.

The long-shot Oakland Athletics winning the World Series would be a costly, worst-case scenario for MGM sportsbooks. The A's were as high as 250-1 to win the World Series in June, and bettors pounced.

"They're the worst, by far," Jay Rood, MGM vice president of race and sports, said of the A's, who take on the New York Yankees in the AL wild-card game Wednesday in New York. "We have just a slew of $5 and $10 tickets at anywhere between 100-1 and 250-1 [on the A's]."

Included in the MGM's slew of World Series bets on the A's are a $1,000 bet at 250-1 and a $1,000 at 125-1, which were each placed in June and would net $250,000 and $125,000, respectively. The Athletics' odds had shortened to as low as 6-1 at MGM ahead of the playoffs.

Oakland also was the best team to bet game-by-game throughout the regular season. If you bet $100 on the A's in each game this season, you'd have turned a $2,800 profit, the best return for any team this season.

The Red Sox enter the postseason as the favorites at 3-1, followed by the Astros at 7-2, at the Westgate SuperBook in Las Vegas.

The Los Angeles Dodgers are the favorites to represent the National League in the World Series. The Dodgers are 5-1 at the Westgate, followed by the NL Central Division champion Milwaukee Brewers at 7-1.

The Red Sox began the season as part of the second tier of contenders at 10-1 to win the World Series. They moved past the Astros into the role of favorite by August and went on to win an MLB-leading 108 games.

Only five of 23 World Series champions during the wild-card era had the best record during the regular season, according to ESPN Stats & Information, but two of the five were the Red Sox in 2007 and 2013.

On Sept. 23, a bettor at the Westgate placed a $20,000 bet on the Red Sox to win the World Series at 4-1.

The Chicago Cubs, Cleveland Indians and Yankees are each 10-1, followed by the Atlanta Braves at 12-1. The Colorado Rockies and A's are each 20-1 at the Westgate.

More money had been wagered on the Yankees to win the World Series this season than on any other team at William Hill sportsbooks in Nevada and New Jersey. The Red Sox had attracted the second-most wagering support.