(These settings are all with expert mode turned on, so remember to turn on expert mode before whinging that you can't find one of these options).

On the Fritz!Box 7390, click "Internet", then "Account Information". Settings as follows:

Connect Via: "Internet connection via DSL"
Operating Mode: "Use one Internet connection for all computers (router)
Account Information: "No account information required (in accordance with RFC 1483 / RFC 2684)"
Connection settings:
VPI: 0
VCI: 101
Encapsulation: Bridged (also tick "Obtain the IP adress automatically (DHCP)")
Select "Set the speeds of the DSL line automatically".

Things to note, these settings work for me, YMMV ;-) These settings are for the O2 Home Broadband LLU product without a static IP. Obviously, if you have a static IP, or use the non-LLU O2 connection, these settings will require modification.

[UPDATE: I added my IPv6 stuff on a new post, Fritz!Box 7390 Hurricane Electric TunnelBroker.net IPv6 tunnel, enjoy]