https://www.gamespot.com/articles/more-original-xbox-backwards-compatibility-games-a/1100-6456421/

There are currently 13 Original Xbox games in the Xbox One's backwards compatibility lineup, including great games like Star Wars: Knights of the Old Republic and Psychonauts. More are coming, Xbox boss Phil Spencer has confirmed, though he did not give anything away about what these titles might be.

"There are a number of [Original Xbox] games in [backwards compatibility] pipeline," Spencer said on Twitter. However, he cautioned that getting OG Xbox games into the catalog is "more complicated" than it is for Xbox 360 titles. Getting permission from the rights-holders for titles is more involved than it is for Xbox 360 titles, he said, which will mean that there won't be as many OG Xbox games in the library as there are Xbox 360.