Yes, you can do this.
No, there is no one sentence answer explaining all the possible ways to do it.
Best way to do this is by associating a person, or a group of people, in your Google contacts, with a specific phone so that people in that group ring both your Obi phone and a cell phone. Everyone else rings only your OBI/Google phone. I'm not exactly sure what you mean by 'outside calls', outside your listed group(s) of known people? This is sort of what Google attempts to model with the groups function.
How to do this? Start here:
http://support.google.com/voice/?hl=en#topic=1708125
and read the short explanations under:
How to > Call forwarding > 'Setting which phones will ring' and 'Call forwarding with your Google number'
and
How to > Voicemail, Contacts, Groups > Contacts, Groups, Greetings
There are probably a few more topics in there but you'll find them.
If you have a specific question, reply back.
