I have never actually done this. I have tried reading a lot of the admin guide. Your posting your experiences will help others.
There are two ways to get to bridge mode: use the local web server on the OBi (not OBiTalk), or use an attached phone.
You can access the local web server via the LAN port, or you can access through the the local web server via the Internet (WAN) port after you have enabled that by phone. ( I suspect you did that already.)
The path is Router Configuration>LanSettings>OperationMode where you can choose Bridge
To put the OBi into bridge mode, via an attached phone dial ***0, 31#, 1,
1#,1.
To put it back to router mode dial ***0, 31#, 1,
0#,1. (comma means pause and listen)
After entering a string, you could just hang up.
http://www.obitalk.com/forum/index.php?topic=5484.0 is some prior discussion.