The option you are looking for is boxvideo, which is available for mpv 0.31 and above (see relevant git commit). Just add:
boxvideo=yes
in your ~/.config/mpv/script-opts/osc.conf and your video should be displayed above the progress bar.
Also make sure to change the vertical position of the subtitles, because they will be probably covered from the OSC now, by adding the sub-margin-y option in your ~/.config/mpv/mpv.conf file as:
sub-margin-y=<size>
where <size> represents the top and bottom screen margin for the subs in scaled pixels.
To install the latest version of mpv, if it isn't included in the default repositories of your Ubuntu version, you can add mpv's PPA by running the following commands:
sudo add-apt-repository ppa:mc3man/mpv-tests
sudo apt-get update
Then reinstall mpv:
sudo apt install --reinstall mpv