The Oval Office is redecorated by every new president and military flags are not traditionally displayed in the Oval Office.
The Oval Office is the President of the United States' formal workspace, where they confer with heads of state, diplomats, their staff, and other dignitaries. According to the White House Museum, "Each president has decorated the Oval Office to suit his tastes. Among the features that remain constant are the white marble mantel from the original 1909 Oval Office, the presidential seal in the ceiling, and the two flags behind the president's desk—the US flag and the president's flag."
Former President Donald Trump had military flags in the Oval Office, which includes the Army and Marines flags. It was positioned on Trump's left side behind the Resolute Desk next to a window. On January 21, 2021, the Washington Post published an article based on its exclusive 20-minute tour of the oval office. They spoke to Ashley Williams, the deputy director of Oval Office operations, who said, "This Oval is an Oval for Day One". She exclaimed, "It was important for President Biden to walk into an Oval that looked like America and started to show the landscape of who he is going to be as president".
The article described the changes and constant of the office. Biden has decorated the room with many portraits and busts, including portraits of Franklin Delano Roosevelt, George Washington, Abraham Lincoln, Benjamin Franklin, Thomas Jefferson, and Alexander Hamilton and busts of Rosa Parks, Eleanor Roosevelt, Martin Luther King Jr., Daniel Webster, Cesar Chavez, and Robert F. Kennedy. The post also reported, "Gone are the flags of the branches of the military that Trump displayed behind the Resolute Desk. Biden has installed an American flag and another with a presidential seal."
The oval office is redecorated according to the newly elected president's choice after the election. Hence Bidden has removed many portraits, busts, and flags, by replacing them with his taste. Moreover, military flags are not traditionally displayed in the Oval Office. During most presidencies, two flags have been displayed behind the Resolute Desk: The American flag and a flag bearing the presidential seal.