My reading is that PADI OW/AOW train you for "no stop" diving and if at any point you have a mandatory deco stop ("yes stop"), you are diving beyond your training. I have a suspicion one might be able to come up with a profile that will have a deco ceiling (i.e. "yes stop") that, if you come up at safe ascent rate, will clear before you get there (i.e. "no stop"). If that's true, that'll be a no-stop dive, no deco, mild or otherwise, and yet beyond OW training. That's the shady area.
There is no shady area there. Any dive that comes close to the NDL will have a ceiling - which you will not hit if you ascend at the rate you're trained to ascend at (probably 30 ft or 10m per minute). If you're diving a computer, "ceiling" or not, if the computer says you have NDL of 0 or greater, then you have not exceeded OW training. Similarly, if you are diving using tables and a bottom timer, as long as you start your ascent at the time the tables said, or less, and ascend at 30 fpm/10 mpm, you still have not exceeded your training.
The people who developed the NDL tables and the algorithms that produce NDLs on a computer knew that diving to your NDL will result in having a ceiling (that you will not hit if you ascend at the correct rate or slower). As long as you follow the NDL, you have not violated OW training.