Microsoft’s Major Nelson confirmed this on Twitter. If you already own the game for Xbox 360, it should show up automatically in your Xbox One game library. If not, you can pick it up right now on your console or through the Xbox Store website. Additionally, the game is included in the Xbox Live spring sale.
On the Xbox Feedback site, Black Ops II received more than 213,000 votes from people asking for it to be added to the backwards compatibility catalog, the most of any game.
Xbox boss Phil Spencer personally advocated for Black Ops II to come to the library and heavily suggested in the past that its addition to the catalog was a matter of when not if.
The Xbox One’s backwards compatibility library has more than 320 games in it–you can see them all here.