Currently there isn't any way to see the available commands.
I can't do onlykey --help.
The python module argparse (which I think is included by default in python), can simplify argument parsing and adds a nicer user experience, by automatically creating an help section.
Are you interested in adding an --help flag?
Can i do a pull request introducing argparse and adding the --help flag?
https://docs.python.org/3/library/argparse.html