With Doug Fister and Wilson Ramos returning from injuries this week, the Post Sports Live crew debates whether the starting pitcher or catcher will have a bigger impact on the Nationals. (Post Sports Live/The Washington Post)

Before Friday night’s scheduled opener of a three-game series, the Washington Nationals and Oakland Athletics hadn’t played each other since 2005 and had never met here at O.co Coliseum. Yet over the past four years, the two infrequent opponents have become inextricably intertwined.

Since 2010, the Nationals and Athletics have made eight trades — the most the Nationals have made with any one team during General Manager Mike Rizzo’s tenure. This past offseason alone, Rizzo and Athletics General Manager Billy Beane exchanged five players in three trades. Players from many of those deals will play key roles for both sides this weekend.

Athletics left-handed starter Tommy Milone, a prospect acquired in the December 2011 trade for Gio Gonzalez, is Friday’s scheduled starter. Derek Norris, also part of that deal, has been one of Oakland’s two primary catchers this season. Gonzalez is slated to take the mound Sunday afternoon. Nationals left-handed reliever Jerry Blevins, acquired this winter, will be called upon to face of Oakland’s toughest left-handed hitters, his former teammates. The same applies for Athletics left-handed reliever Fernando Abad.

“We just fit on a lot of different deals,” Rizzo said.

How the Nationals and Athletics became frequent trade partners is a combination of coincidental need and relationships. Rizzo, 53, is a former longtime scout who rose through the ranks and was hired as the Nationals’ full-time general manager in August 2009 after a stint as the interim GM. Rizzo shepherded the Nationals from a bottom-dweller to a contender with a strong emphasis on scouting and drafting.

The Post Sports Live crew debates whether Bryce Harper's intense style of play attributed to his hand injury. (Post Sports Live/The Washington Post)

Beane, 52, also a former player turned scout, ascended in the Athletics’ front office and was named general manager in October 1997. Beane, whose embrace of advanced statistics was made famous in the book-based movie “Moneyball,” has built the Athletics with a small payroll by relying on trades.

Over years of crossing paths and talking, even when Rizzo was with the Arizona Diamondbacks, Rizzo and Beane built a working relationship and mutual respect.

“I think a lot of people underestimate the need for relationships between the two principals in the deal,” said longtime Athletics assistant general manager David Forst, who assists Beane in player acquisitions. “In this case, Billy and Mike, and they have a very good relationship. It lends itself toward conversation in general, and that’s sort of where trades come from: from everyday casual conversation.”

Trade talks are inherently an uneasy process. One team will be naturally suspect of the other team: Why do they want to trade this player? What do they know about our player that we don’t? Some teams simply don’t talk to each other. Others do so infrequently. But once the decision-makers of two teams have built trust, the casual conversations that lead to informal trade talks are seamless.

“We speak the same language,” Rizzo said. “We’re both very honest and upfront with each other. . . . If you don’t have a match, it’s not a drawn-out process. We usually cut to the chase and get things done.”

While Rizzo speaks highly of Beane, he has said in the past that his relationship with him is “a little overstated.” Although Rizzo has a good working relationship with Beane, he is close with Oakland’s director of player personnel, Billy Owens. Rizzo and Owens know each other from years of competing against each other as scouts, and trade talks have started between them, too.

“Every deal that we ever make starts with people that work for us,” said Rizzo, who boycotted the movie “Moneyball” because he disagreed with the depiction of scouts. “You always have your scouts and your special assistants out and about and they’re always talking. There are messages carried from the GM to the guys out in the field and there are communications. Billy Owens is a guy that I’ve known for a long time also and a good baseball guy and can carry a message back to Billy Beane.”

The first trade between Rizzo and Beane was on Dec. 16, 2010, when the Nationals sent power-hitting left fielder Josh Willingham, who had one year remaining on his contract, to the Athletics for two prospects, fire-balling reliever Henry Rodriguez and outfielder Corey Brown. Willingham spent one season in Oakland, and Rodriguez and Brown are no longer with the Nationals.

Since then, the Nationals and Athletics have pulled off blockbuster deals (Gonzalez), multi-team swaps (the Michael Morse deal) and smaller trades (Kurt Suzuki twice and Abad).

“It just so happens that they had players that we wanted and fit what we were trying to do and the same thing in return,” Rizzo said. “We had good communication and things that made sense to both organizations. We’ve done deals with a lot of teams, but Billy is a guy that I’ve known for a long time and have dealt with for a long time. I think it’s less about personality and more about communication. He’s direct, and there’s not a lot of things going on.”

The deals between the two teams also have been fair. The Nationals have benefited from Gonzalez, Blevins, Blake Treinen and, for one season, Ian Krol, a player to be named later in the Morse trade that they used to acquire starter Doug Fister from Detroit. The Athletics are happy with Milone (starting depth), catcher John Jaso (acquired from Seattle in the Morse trade), Norris, Abad and minor league outfielder Billy Burns (impressive in major league spring training). It certainly helps that the teams are in different leagues and don’t face each other often.

“It seems that every trade we’ve done has had legs on both ends,” Forst said. “. . . Billy has said this publicly: Our goal in trades is not to screw the other party because then you’ve lost one of 29 trading partners going down the road. You want things to work out so you continue to have that relationship.”