Yes, you only need to edit the firewall:
config zone
option name 'lan'
list network 'lan'
list network 'vpn'
option input 'ACCEPT'
option output 'ACCEPT'
option forward 'ACCEPT'
config rule
option name 'OpenVPN'
option src 'wan'
option proto 'udp'
option dest_port '1194'
option target 'ACCEPT'
Source (Czech only): OpenVPN - konektivita mezi VPN a LAN
You may still need to edit /etc/config/network from here: http://www.s474n.com/project-turris-zprovozneni-openvpn-serveru/