Original Xbox titles have started to show up on the Microsoft store for digital purchase

UPDATE:
Microsoft announced the first 13 backwards-compatible original Xbox games for Xbox One. Here is the full list:

• Star Wars: Knights of the Old Republic
• Ninja Gaiden Black
• Crimson Skies: High Road to Revenge
• Fuzion Frenzy
• Prince of Persia: The Sands of Time
• Psychonauts
• Dead to Rights
• Black
• Grabbed by the Ghoulies
• Sid Meier’s Pirates!
• Red Faction II
• BloodRayne 2
• The King of Fighters Neowave

These first 13 will be available tomorrow, October 24. All are enhanced with 1080p resolution, higher and/or smoother framerates, and faster loading times.
Original: During Microsoft’s E3 conference this year one of the last announcements was Original Xbox Backward Compatibility. It has been by far one of the most requested features to come to Xbox One and while many thought it was impossible, the team at Xbox have made it happen again.
It’s expected to come this Fall and the company have yet to say anything official about what games will be there at first besides Crimson Skies. Well according to the listing on the Microsoft Store there are six games we do know that will be coming to the program at first.
You can expect Star Wars – Knights of the Old Republic, Prince of Persia The Sands of Time, Red Faction 2, KOF NEOWAVE, BloodRayne 2. and Dead to Rights. They are all listed between $10-$15 digitally, but if you do have the games physically you can pop it in and install on your Xbox One just like with the 360 Backward Compatibility.

You can see the listing here.