I am not a lawyer (hooray!) but I dug a bit into the Antiquities Act, which controls the creation of National Monuments. The authority for the President to do so is in 54 U.S. Code Subtitle III Division C Chapter 3203, but the justification is in Chapter 3201, which includes
It is declared that it is a national policy to preserve for public use historic sites, buildings, and objects of national significance for the inspiration and benefit of the people of the United States.
So if Trump has declared the Oval Office to be a National Monument, that says we should all be able to use it for meetings, no?