Equivalent to "source" in OpenBSD?

You are using the Forsyth PD Korn shell, the usual login shell on OpenBSD. The PD Korn shell does not have a source command. The source built-in command is only available in some shells. The command that you want is the . command.

Further reading

• What is the difference between '.' and 'source' in shells?

The source keyword which is available in bash is not part of the Posix standard. Instead you can use

. myVenv/bin/activate

You could use the same syntax with . in bash which you are using on your Linux system.