
As users "own" their ODFB site collection, they should be able to access the second-stage recycle bin too. Go to Recycle bin, if the items you want to restore are not listed therein, scroll down to the bottom of the page and hit the Second-stage recycle bin link. Alternatively, you can open it directly via the /_layouts/15/onedrive.aspx?view=13 URL. If files were deleted from there too, you cannot recover them (you can try opening a support case though, as Microsoft have some limited capabilities to recover such files).
In addition, if the user is subject to hold/retention policy, the files might be accessible within the Preservation Hold Library.