You can install a wall oven under the counter, as long as you realize it will be a bend down lower to access, and you follow all the cut sheet and installation requirements.

It does not require venting in the counter top, nor behind. That's why they are "wall ovens": )

You read the install directions I have no idea wahy anyone wants to install a wall oven under a counter they are called wall ovens for a reason. If you want one low get a range with the oven at that level. Wall ovens amade to not require extra venting but there are always instructions for install.