As far as I know, there's no way to get Private Owner Key from the Steemit website, most likely because it would be additional security risk.

"The private key or password for the owner key should be kept offline as much as possible"

However, for people that are using Master Password there's no much difference and no bigger risk (leaking Master Password is same as leaking all the keys, including Private Owner Key).

You can use get_private_key_from_password prinzvalium owner YourMasterPassword in cli_wallet (if you can manage to run it) to get your Private Owner Key from your Master Password, or just stop using Master Password (keep it offline, keep it secret, keep it safe), and just use Posting Key for day-to-day use cases, and Active Key only when you need to transfer funds or approve/unapprove witnesses etc.