Title says it all.
If I host my own VPN at home (using openvpn), and connect my devices to it, will my ISP be able to see the contents of my traffic?
Your setup is not fully clear. But given the description I assume that you create your own OpenVPN server and use it as an VPN endpoint when you are outside of your local network. Any connection to the internet from the client will then send protected by the VPN and invisible to the ISP to your home. At your home the traffic will then leave the VPN (since your home is your VPN endpoint) and will be send without further protection from your home to the internet - i.e. the ISP can see where on the internet your traffic is going and in case of unencrypted traffic can also see the contents.
No, the traffic will pass over their network as normal
Using a VPN (even one you host yourself) should protect you from monitoring on public internet connections, assuming you don't have leaks such as DNS leaks, which can lead to information being revealed.
It depends which traffic you are talking about. If you are away from home and you connect to the VPN, the contents of the traffic between you and the VPN server will not be visible. However, if the VPN is configured as your default route for internet bound traffic, that traffic will pass through the VPN and leave your home destined for the internet, where it will no longer be encrypted. At this point, your ISP can see what is leaving the house.
This is all assuming OpenVPN is configured to use encryption, which is usually the case.