AirVPN, a commercial VPN provider, explained many of the technical challenges, risks, and benefits of using the two technologies together. I would only recommend using them together if you are a technical user and/or want to experiment.
If you want to use them together, I would recommend using a separate device for your OpenVPn connection, such as OpenVPN running on OpenWRT, pfSense router, OnionPi, (or similar) so that your PC can't get it's OpenVPN and Tor client connections screwed up. This corresponds with the section Using Tor over AirVPN
of the AirVPN article.
From the article:
Using AirVPN with Tor
Advantages
Additional privacy layer: our VPN server will not see your real IP address but the IP of the Tor exit node (you can check your Tor exit
IP in the Client Area)
Option to connect to web sites under Tor protection, even to those sites which refuse Tor connections
Access to Tor from all the applications transparently: no need to configure each application, one by one
Support to non-TCP applications which can not be supported by Tor
Access to AirVPN DDNS
Access to AirVPN DNS and micro-routing against IP address-based geo-location discriminations
Access to Remote Port Forwarding
Avoidance of any traffic discrimination from Tor exit nodes (packets are still encrypted when they pass through Tor exit node)
Major security layer in the event you pass through a compromised/malicious Tor exit node (packets are still encrypted when
they pass through the Tor exit node)
Disadvantages:
Poor performance
Fixed Tor circuit for each OpenVPN session
Access to .onion sites only from browsers configured to connect directly to Tor
Notes
Browsing with the Tor Browser, or running any application configured to use Tor Socks, generates traffic that's always directed
to the Tor network and OUTSIDE the VPN tunnel. Technically because
they use a connection that had been established before the VPN
connection started.
If you use the Tor Browser to reach https://airvpn.org, the bottom box will always show a red 'Not connected', with an IP address of a
Tor Exit Node. This is because Tor browser enters directly the Tor
network. If you use another browser (not configured to use Tor socks
proxy), you will see the correct green box displaying 'Connected!'.
Our client automatically works with the default torrc configuration file included in the Tor Browser Bundle. If you change
something, for example Socks/Control ports or control authentication
settings, you need to update AirVPN client options accordingly. In the
torrc default configuration file of different packages, minor but
essential modifications may be necessary.
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Using Tor over AirVPN
Using Tor over AirVPN If you wish to connect over Tor over AirVPN:
Connect normally to an Air server, in any mode except Tor mode
Launch Tor after the connection to a VPN server has been established.
All the applications configured to connect over Tor will connect over
Tor over AirVPN. All the applications not configured for Tor will
simply connect over the VPN. Features:
Our servers can see your real IP address.
Our servers can not see your traffic content, real origin and real destinations.
The Tor entry-node will not see your real IP address, it will see the exit-IP address of the Air server you're connected to.
Your are not protected against malicious Tor exit nodes if you send/receive unencrypted traffic to/from the final host you connect
to.
Please note that, with the above setup, if you connect to our web site
with a Tor configured browser, our web server will see your Tor exit
node IP address, so the site will display a red bottom box, as if you
were not connected to an Air server.