You try an nfs share, if your nas provides this? Then you can you add the share via browse and network file system (nfs). This should provided better throughput than non fstab mounted samba shares.
Failing that, I think the following should get the share mounted via fstab; more details about your nas would help.
You will need access to the command-line
sudo mkdir /mnt/video
sudo nano /etc/fstab
Add this line at the bottom, update your nas.ip and name of share.
//nas.ip./nameofshare /mnt/video/ cifs noauto,systemd.automount,ro,uid=1000,gid=1000,iocharset=utf8 0 0
If you are using user and password you will need to add to this entry, searching the forums will help with this.
Now reboot osmc. Now if you try add a new source and browse to root file sytem, mnt and video, and you can see your shares; then it has mounted correctly.