I'm somewhat reluctant to respond on this as I won't be able to give precise step by step instructions. My experience with Asterisk/FreePBX is limited and not doing what you are seeking. However, some guidelines.
1. Don't try to get it all working in one shot. Build it up bit by bit, testing each bit as you go along.
2. Use fixed IP addresses for the OBis and the PBX.
3. Configuring the OBis should be relatively straightforward. There are 3 aspects:
3.1 If you want 911 calls to not go through the Asterisk then you need another Service Provider for that. So you need to configure 2 Service Providers and 2 Voice Services. (SP1 for Asterisk and SP2 for the VoIP SP you are using for 911.)
3.2 You need to think about numbering so that you can craft dial plans which route outgoing calls 2 ways on the OBis and 2 (or more?) ways on Asterisk. The OBis need to recognise 911 calls and send them to SP2 while all others go to Asterisk on SP1. Asterisk needs to differentiate calls to other extensions (OBis) from calls to external numbers.
3.3 As far as Asterisk is concerned the OBis (except 110) are extensions and are configured on Asterisk in the normal way with extension number and 'secret'. Configure the OBis with the same number and secret (Voice services/SP1/SIP credentials) to register with Asterisk. Use the IP address of your Asterisk box for the field at Service providers/ITSP Profile A/SIP/proxy Server.
With the above, you should be able to make calls between the OBis and make 911 calls.
Next task would be to the PSTN line (in and out) operational. This is where I'm getting out of my depth as I have no first hand experience with that.
Have a good read of
http://www.obitalk.com/forum/index.php?topic=57.0 and
http://www.obitalk.com/forum/index.php?topic=1157.0 .