@oldtimer, @direwolf: No, they are not the same.
Owner Key is derived from the Master Password in a same way as Posting Key.

Let's say user `bob` is setting his new Master Password.
It will look like this: `P5Hzer2h4R4Lkkjr455T4msnJyjwwmrjLLDYNATMAukM2yehVE6R`.
Steem blockchain however, doesn't know anything about Master Password as such.
It uses keys, such as Posting Key, Active Key, Owner Key that are **derived** from the Master Password.
In such case `bob` will have:
Private Owner Key:
`5JiD4BEytbFWMGeN3Zk9JfFFgFCTvfcDhDGReG7jt2DREY8JzMa`
Private Active Key:
`5K6p5g2ob577bA53qgLMGDGY3L3D7M4ccaY2qFSJppgEvJkeLFn`
Private Posting Key:
`5KW5yYgmPf7bRn6BFEWboLr9bj4QtmVJMNAm2SiErDN5BCGtWH5`
How I know it? There's a `cli_wallet` functionality that lets you derive key pairs from the Master Password. It's used for convenience, as you need to securely store one, instead all four.
To derive Owner Key from Master Password, `bob` would need to use:
`get_private_key_from_password bob owner P5Hzer2h4R4Lkkjr455T4msnJyjwwmrjLLDYNATMAukM2yehVE6R`
Same for every role.

In fact you don't need to have Master Password at all. Your private keys can be generated and changed independently.