Darren O’Day should be a coveted arm. (Nick Turchiaro-USA TODAY Sports)

The best bullpens (see: Royals, Kansas City) aren’t usually assembled through free agency. This is the most volatile market in baseball, and while several free-agent relievers have earned the right to multi-year deals, there’s not a general manager in the game who feels particularly comfortable with them.

Last year, former Yankees closer David Robertson signed for four years and$46 million with the White Sox — and blew seven saves while posting his highest ERA since 2010. Lefty Andrew Miller got four years and $36 million to replace Robertson in New York, and was superb, converting 36 of 38 opportunities while striking out 100 hitters in 61 2/3 innings.

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Those mega-deals won’t be approached this offseason, when there is only one established closer available. (Next year’s class, barring contract extensions, would include Pittsburgh’s Mark Melancon, Kenley Jansen of the Dodgers, Arizona’s Brad Ziegler and Washington’s Jonathan Papelbon and Drew Storen.) Here are a half-dozen to look at this winter:

6. Tony Sipp, left-hander: When Sipp arrived in Houston before the 2014 season, he had a marginal 3.86 ERA and, more important for a potential lefty specialist, an average of 4.6 walks per nine innings. But he found himself with the Astros and is coming off a season in which he posted a 1.99 ERA and struck out 62 batters in 54 1/3 innings. Though he’s excellent against left-handed hitters (he allowed one homer and five doubles in 108 plate appearances against them), he can retire right-handers, too; they hit just .190 against him in 2015. He is, too, durable: 10 of his outings lasted more than one inning. The Astros could certainly use him back, but he’ll receive interest from all sorts of suitors.

5. Ryan Madson, right-hander: One of baseball’s best stories in 2015, Madson, the one-time Phillies’ closer, returned from injury and a three-year absence from the majors to appear in 68 games for the Royals, posting a 2.13 ERA and allowing 0.963 walks and hits per inning pitched. Madson’s responsibility increased when Kansas City closer Greg Holland was lost to Tommy John surgery, and he ended up appearing in three World Series games in which he didn’t allow a run. His closing days have passed by, but at 35 he’s a major leaguer again and could get a two-year deal.

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4. Antonio Bastardo, left-hander: If you need a guy to retire lefties, here’s your man. The former Phillie spent 2015 with the Pirates, and left-handed hitters managed only a .138 average, .233 on-base percentage and .215 slugging percentage against him. His career slash line against lefties: .178/.277/.319. He does walk more than four hitters per nine innings, and 22 of his 66 appearances lasted less than an inning, so he’s best suited as a specialist. Still, that could yield a three-year deal.

3. Joakim Soria, right-hander: The only available reliever who served as a closer in 2015, Soria will seek the same role again even though his midseason trade from Detroit to Pittsburgh left him as a set-up man for Melancon. Since returning from Tommy John surgery that cost him all of 2012, Soria has posted a 2.99 ERA and 1.106 WHIP with 140 strikeouts in 135 2/3 innings. The Tigers, whose bullpen seems in eternal flux, could well want him back. It will be interesting to see whether a team signs him as a full-time closer or strengthens its depth with Soria as a setup man.

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2. Tyler Clippard, right-hander: A fascinating case, and a reason why it might be wrong to raise your son as a reliever. From 2010-15, Clippard was one of the most reliable bullpen arms in the game. No pitcher appeared in more than his 440 games (an average of 73 per year), and no reliever threw more than his 464 1/3 innings. His numbers during that run: a 2.67 ERA and 1.032 WHIP to go along with two all-star appearances and a 2012 season when he filled in for an injured Storen with the Nationals and saved 32 games. But there are chinks in the armor here. Clippard allowed runs in three of his eight postseason appearances with the Mets, and in a loss in Game 4 of the World Series, he gave up two crucial walks in the Royals’ game-winning rally. Still, his changeup allows him to hold left-handed hitters to a .137/.231/.237 slash line, and he has established himself as an elite setup man. That won’t bring Miller-type years or dollars. But three years seem certain — and could he get $6.5 or $7 million per?

[Tyler Clippard’s journey from the Nats to the A’s to the Mets ends in the World Series.]

1. Darren O’Day, right-hander: Four years ago, the Orioles plucked O’Day off waivers from the Rangers, and the payoff couldn’t have been bigger. From 2012-15, he posted a 1.92 ERA and 0.939 WHIP, among the best numbers of any reliever in the game. He picked his walk year to have his best year, a 1.52 ERA and 0.934 WHIP over 68 appearances, and he was murder on right-handed hitters, allowing a slash line of .192/.236/.256. He can throw multiple innings; 11 times he got more than three outs. The drawback? At 33, a four-year, Miller-type deal would bring him to 37. But it’s hard to imagine O’Day’s haul won’t be the most lucrative among relievers this winter.