Well your traffic is encrypted (assuming OpenVPN), so all your ISP should be able to see is the amount of data to and from your VPN.

They can of course see the individual packets and their exact timings. This can be trivially correlated with egress traffic from your VPN gateway to other ISP affiliated networks or web properties. And since it's your private gateway, there is no other traffic mixed in.

Traffic analysis is a term of art in cryptanalysis and SIGINT, in case you were not familiar: https://en.wikipedia.org/wiki/Traffic_analysis

I was unaware. Thanks for the info!