The OpenVPN protocol makes no attempt to hide its existence, although using static keys will make the traffic indistinguishable from random data, at least in theory. The protocol itself is briefly described at the end of the [documentation](https://openvpn.net/index.php/open-source/documentation/security-overview.html). Note that, even if you manage to [obfuscate](https://community.openvpn.net/openvpn/wiki/TrafficObfuscation) the protocol itself, it is possible for your ISP to determine that you are using a VPN using [traffic fingerprinting](https://link.springer.com/chapter/10.1007/978-3-642-35795-4_56). Not to mention, the fact that you are using some sort of tunnel can be inferred from the behavior of only connecting with an encrypted protocol to a single IP address for all of your internet activity.