Interleague play has ensured the rivals have faced one another in each year of their co-existence. The Orioles lead the Beltway Series by year, 6–1–3, and the Orioles also lead the overall series, 33–23.
Although this rivalry is new, it has a strong basis because of the circumstances surrounding the Nationals' founding. Peter Angelos, the owner of the Orioles, opposed the move of the Montreal Expos to Washington, D.C. on the grounds that it would harm the Orioles financially, contending that the Orioles alone had a legal right to the Baltimore-Washington market. Ultimately, the owners of the other MLB teams approved the move to Washington in a 28–1 vote on December 3, 2004; Angelos cast the sole dissenting vote.
In the four full seasons since baseball returned to Washington, the Nationals drew 9,127,252 fans to their games, compared to 8,892,951 fans attending Orioles games. The Nationals were a larger draw in 2005, while both teams were about the same in 2006 (the Orioles drew exactly 100 more fans that year), and in 2007 the Orioles were a larger draw by nearly 200,000 fans. In 2008 the momentum swung back the Nationals' way, with the Nationals outdrawing the Orioles by over 370,000 fans, mostly due in part to their then-new ballpark.
The dispute with Angelos over the move was resolved when the Orioles were granted the right to broadcast Nationals games on their new television network, the Mid-Atlantic Sports Network. However, the deal was struck only days before the beginning of the 2005 season and many D.C.-area fans did not receive the new network for some time and were unable to watch most games. Furthermore, it has been reported that Angelos and the Orioles hold majority ownership in the network, which has led some Nationals fans to contend that the channel exhibits a bias in covering the Orioles compared to the Nationals (such as displaying the scores of Beltway Series games as "Orioles vs. Washington" as opposed to "Orioles vs. Nationals"). Furthermore, the Orioles changed the name on their away jerseys from "Baltimore" to "Orioles" in 1972 after D.C. lost the last Senators team, in an attempt to convince D.C. fans to adapt them as their team. In 2009, the Orioles changed back to "Baltimore" on their away jerseys.
The tradition of yelling "O!" during the line "Oh say does that Star Spangled Banner yet wave" in the "Star Spangled Banner" is a Baltimore Orioles tradition. Over the years it became a staple of Washington, D.C. area sporting events. The chant has had controversy in the past and the usage of the "O!" chant at Nationals games has reignited such controversy.