There are currently 41 original Xbox games backwards compatible on Xbox One and if you have an Xbox One X they can run up to 16x their resolution at 1920p HD.
The Xbox One released November 22, 2013 and was followed by a revision console by Microsoft name Xbox One S released August 2, 2016. The latest revision is the Xbox One X which released November 7, 2017. These three versions of the Xbox One have backwards compatibility functionality with original Xbox but each revision takes advantages of scaling up the resolution. To quote wikipedia "All Original Xbox games run at 4 times the original resolution on Xbox One and Xbox One S consoles (up to 960p), and 16 times on Xbox One X (up to 1920p)."
When the last batch of original Xbox game backwards compatibility titles were announced for the Xbox One Phil Spencer released this statement,
"Making sure that all four generations of content--so the original Xbox games that run on your Xbox One today, the OG Xbox; the 360 games that run on your Xbox One; your Xbox One games; and the new generation games--all run on the next platform is important to us." - Phil Spencer
I think that this statement has been misunderstood by fans across all generations that the Xbox Scarlett is going to be backwards compatible with every Xbox game before it. I think what Phil Spencer was saying, is that all the "current" backwards compatible games will also be compatible on the Xbox Scarlett. Not to be confused with the idea is that more original Xbox games will become backwards compatible but that the current backwards compatibility on the Xbox One will transfer over.
Xbox One backwards compatibility for the original Xbox started October 24, 2017 with just a handful of games. There are now 41 backwards compatible original Xbox games for Xbox One. See image below for higher resolution. Oddly enough the Xbox website decided to leave out Grand Theft Auto San Andreas from the visual list.
- Armed and Dangerous
- Black
- Blinx: The Time Sweeper
- Bloodrayne 2
- Breakdown
- Conker: Live and Reloaded
- Crimson Skies
- Dead to Rights
- Destroy All Humans!
- Full Spectrum Warrior
- Fuzion Frenzy
- Grabbed by the Ghoulies
- Grand Theft Auto San Andreas *not pictured above
- Hunter: The Reckoning
- Indiana Jones and the Emperor's Tomb
- Jade Empire
- King of Fighters: Neowave
- Mercenaries: Playground of Destruction
- MX Unleashed
- Ninja Gaiden Black
- Panzer Dragoon Orta
- Panzer Elite Action: Fields of Glory
- Pirates!
- Prince of Persia: The Sands of Time
- Psychonauts
- Red Faction 2
- Sphinx and the Cursed Mummy
- SSX 3
- Star Wars Battlefront II
- Star Wars Battlefront
- Star Wars Jedi Knight: Jedi Academy
- Star Wars Jedi Starfighter
- Star Wars Knights of the Old Republic II: The Sith Lords
- Star Wars Republic Commando
- Star Wars: Knights of the Old Republic
- The Elder Scrolls III: Morrowind
- Tom Clancy's Splinter Cell Chaos Theory
- Tom Clancy's Splinter Cell Double Agent
- Tom Clancy's Splinter Cell Pandora Tomorrow
- Tom Clancy's Splinter Cell
- Unreal Championship 2: The Liandri Conflict
source: https://www.xbox.com/en-US/xbox-one/backward-compatibility, https://www.eurogamer.net/articles/2019-04-10-xbox-one-backwards-compatibility-list-xbox-360-games-6011
