https://www.gamespot.com/articles/xbox-one-adds-new-original-xbox-backwards-compatib/1100-6458121/

After Xbox One backwards compatibility support for original Xbox games was announced, Microsoft made it clear that we would not see new titles added regularly. To date, only one batch of games has been released, but another has now arrived--and it includes some quality additions.

Until now, there had been 13 original Xbox games available on Xbox One through backwards compatibility. Those include Black, Psychonauts, Prince of Persia, and Star Wars: Knights of the Old Republic. That's in addition to the hundreds of Xbox 360 games that are also playable, such as recent addition Call of Duty 4: Modern Warfare.