It looks like you need some systemd "magic" to successfully mount an NFS share on OSMC. Adding "x-systemd.automount,noauto" to the mount options in /etc/fstab did the trick for me.
Despite having read the opposite, it looks like IPv6 is disabled by default on the latest OSMC release (2015.06-1). I've tried adding a sysctl.d file to set /proc/sys/net/ipv6/conf/eth0/disable_ipv6 to 0, but this did not help. It worked only when I ran the command manually. By going through numerous forum posts I eventually found out that you need to use the connmanctl CLI tool: