1) Allow the relevant IP or IP Blocks to have NFS access. Do this through your control panel.
2) Find the actual name of the NFS export on the backup server:
showmount -e [theftpserver.ovh.net]
3) Create the mountpoint:
mkdir /backup
4) Mount to it:
mount -t nfs -o _netdev,mountproto=tcp [server]:[expert path from 2] /backup
5) You should now see the space available:
df -h
To make this permanent and survive reboots edit /etc/fstab and add:
[server]:[export_path_from_2] /backup nfs _netdev,mountproto=tcp 0 0