SSH and OpenVPN are unrelated services, you can use any of these at any time.
That OpenVPN is a client I suppose?
In this case if "redirect-gateway" is in use, all outgoing traffic from the point of view of the Pi goes to the tunnel, arrives at the VPN server and goes to its destination from there.
If you hit the address of your home router, the Pi receives the SSH connection, and sends its responses (and everything with a non-local destination) via the VPN server. Your computer never sees answers coming from the IP address of your home router as it expects. Meanwhile it is "attacked" by weird traffic coming from a foreign IP, the VPN gateway's IP.
If the VPN server's internet gateway allows it, you could direct an SSH connection to its IP, and the gateway would forward the connection, through the tunnel, to the Pi. It would respond through the tunnel as well, and all would be fine.
Or perhaps you can do away with redirect-gateway and run a "split tunnel" setup, where the Pi keeps its normal gateway address, and only uses the tunnel to reach other peers on the other side.
That is the way many site-to-site VPNs were setup in the old days, performance and availability is better; This setup is less secure/paranoid and most of all it does not "hide my *ss", so full tunnels are all the rage now.
Simpler solutions would be a.) to SSH into another host on the LAN, and from there SSH into the Pi (local traffic does not go via the tunnel), b.) run a VPN client instance on your machine, "meet" the Pi over the VPN and SSH into it.
"S'il n'y a pas de solution, c'est qu'il n'y a pas de problème." Les Shadoks, J. Rouxel