During this year's E3 conference, Microsoft announced that it would be adding compatibility for original Xbox games in order to make them playable on Xbox One. The only game announced to be playable so far is Crimson Skies: High Road to Revenge. Microsoft employee Albert Penello stated on Twitter that physical Xbox discs would work on Xbox One and that digital game licenses would carry over. Microsoft announced that this feature would be released later in 2017. During the conference, they also mentioned that games would run better on Xbox One.
Last year, Microsoft added Xbox 360 backwards compatibility onto the Xbox One. Currently, more than 300 Xbox 360 titles are playable on Xbox One including Mass Effect 1-3, Call of Duty Black Ops, Red Dead Redemption, Halo Reach, etc. A full list can be found at http://www.xbox.com/en-US/xbox-one/backward-compatibility/available-games