For mileage upgrades, you will need "OY" space (for business-class coded flights).
This is not the same as general "O" space that you can find on Delta.com, and you cannot see this inventory unless you call in.
I will also mention that unless you are short on miles, it's generally quite expensive to buy a Y/B/M/H/Q/K-fare ticket and then upgrade.... IIRC H/Q/K require something like 60-80k miles to upgrade one way between US and Europe? Whereas a low-level award ticket outright will require 70k miles.
So buying a Y/B/M/H/Q/K ticket requires the cash ticket, plus another 60-80k miles, while just buying an award ticket with miles may cost less. Your experience, of course, can always vary depending on the dates you are looking to go.