Combining Tor over OpenVPN provides you with remarkable benefits. Some examples:
you tunnel efficiently UDP, which Tor alone can't handle
you tunnel at least over the VPN any system process with high privileges binding without your knowledge
you hide your Tor traffic to your ISP and government (really relevant but only in some countries)
you exit from the VPN server to enter a Tor circuit (Tor circuits are re-built normally, the fixed circuit problem is relevant in OpenVPN over Tor)
you can split traffic to balance load, aggregate bandwidth etc.
you can use protocols which are not welcome, not recommended, not usable or too sluggish on Tor network (one example on the next point)
you can use BitTorrent (and any other software which behaves similarly or relies on STUN) without risking your real IP address is revealed, as it may happen (and it happened) with Tor alone https://blog.torproject.org/bittorrent-over-tor-isnt-good-idea?page=0
This is false with Tor over OpenVPN for very obvious reasons (speaking of which, if it was true then the correlation would be absolutely identical and successful with your ISP IP address!).
With OpenVPN over Tor of course you have a fixed circuit because Tor does not change circuit for the same TCP stream and that's an issue to seriously consider. Therefore OpenVPN over Tor may be a starting point to use Tor over itself and establish "dynamic" circuits (a new one for each stream) over a VPN tunnel over a fixed Tor circuit. In this way you have all the advantages given by Tor while our VPN servers do not come to know neither your real IP address nor your real traffic origin and destination (the price to pay is another performance hit). Anyway use it only if you understand perfectly what you are doing, otherwise rely on Tor over OpenVPN and forget about OpenVPN over Tor.
That's the most astonishing thing since sliced bread. 😱 Anyway it is exactly what would happen after a Tor exit node, or after your ISP nearest DSLAM, just to say, if you hadn't end-to-end encryption. The external, first encryption layer of Tor or OpenVPN or your router MUST be wiped out, otherwise how would the final recipient understand your data? By the way HTTP is disappearing so it does not apply much to web traffic. We think that nowadays lack of end-to-end encryption should not be tolerated, and actually we see important steps toward that.
That said, we strongly support Tor (during 2018 and 2019, more than 2.5% of the global worldwide Tor traffic transited through exit-nodes financed by us) and we recommend to use it with and without VPN. Tor network access remains totally free for anyone especially thanks to those people who run at their expenses (money, time, legal issues) Tor exit nodes, just like AirVPN staff does.