The signing: According to industry sources, the Baltimore Orioles have agreed with free-agent reliever Darren O'Day on a four-year, $31 million deal, pending medical review.

The reason: O’Day is good at pitching. Like really good. Since coming to Baltimore in 2012, the submarining righty has the third-lowest ERA in the American League (1.92) and the fourth-lowest WHIP (0.94), and has developed into one of baseball’s premier setup men. He’s also one of the most durable, appearing in at least 68 games in each of the past four seasons, not to mention a well-respected clubhouse leader.

O’Day’s wife is a Fox News correspondent based in D.C., and it was no secret that he wanted to stay in the Baltimore/Washington area. The Nationals, who are in dire need of bullpen help and whose $174 million payroll was $56 million higher than the Orioles' last season, were considered the favorite to sign O’Day, the top free-agent reliever on the market. But according to reports, the Nationals weren’t willing to give O’Day, who turned 33 in October, a four-year deal. Ultimately, the O’s were.

The impact: Over the past four seasons, thanks in no small part to O’Day, Baltimore relievers have pitched to a 3.21 ERA, second in the AL behind the Royals. This past season, O’Day was especially filthy, going 6-2 with a 1.52 ERA, and even earning an All-Star nod for the first time in his career. With closer and fellow All-Star Zach Britton behind O’Day, and emerging relievers Brad Brach and Mychal Givens in front of him, and with K.C.’s recent bullpen changes -- closer Greg Holland had Tommy John surgery in October and Ryan Madson has reportedly signed with Oakland -- the Birds’ pen suddenly looks like the most dominant group in the AL.

The flip side (there’s always a flip side) is that by resigning O’Day, the Orioles' payroll is starting to swell to the point that keeping Chris Davis in town seems nearly impossible. Assuming there’s no deferred money involved in O’Day’s new deal and that his 2016 salary will equal the average annual value of the contract ($7.75 million), the Orioles now have $69 million committed to next season. Add to that the $39 million in projected salaries for their arbitration-eligible players, and they’re already at $108 million. And that doesn’t include another $10 million (at least) that they’re likely to pay whichever starter they sign to replace Wei-Yin Chen. They also could use another outfielder or two, so let’s call that $5 million to $10 million more. That puts the team's projected payroll in the $120 million range, and that’s without Davis, who’s slated to cash in to the tune of $20 million a year, give or take. If the Orioles re-sign Davis, that would put their 2016 payroll in the neighborhood of $140 million, if not more. For a spendthrift club that last season was 13th in payroll at $118 million (a franchise record), that’s an awfully big number.

In other words, Mark Trumbo had better make sure that his first baseman’s mitt is all oiled and ready to go.