You can't. Its a limitation in Unity.
Previously the sound space was defined in the sound's Import Options. Since it is now tied to the Audio Source and PlayOneShot does not return an Audio Source to manipulate then its not possible at the moment. I'm not sure if they realized the ramifications of making that change.
I talked to Richard at Unity about this, not sure if its going to move forward any time soon. It seems like PlayOneShot is almost useless now so the solution is basically to just stop using it.
The better workflow is to add Audio Sources to anything playing a sound, then tell the Audio Source to Play as needed.