How does OpenVPN work?
OpenVPN uses two different protocols. For the standard and most common settings, a TLS link called the control channel is used to exchange a symmetric key. This symmetric key is then used to encrypt a second protocol, the data channel, which uses the OpenVPN protocol itself. The protocol is not identical to IPSec, but it is based off of IPSec ESP, a protocol for secure transport of data over the UDP protocol, which plain TLS cannot do (at the time the protocol was created, DTLS, which is TLS over UDP, did not yet exist). Note that this means that you may be using multiple ciphers for encryption. You may find the control channel using AES to exchange a key that will be used to encrypt the data channel with Blowfish.

Using the control channel with TLS is not the only way to key the data channel. You can also configure OpenVPN to use preshared static keys which will directly be used to derive the symmetric key for data channel encryption. This loses forward secrecy, but gets around the need to use the control channel to exchange a key using TLS.