The 2012 extension

There's a detailed walk-through here: londonconnections.blogspot.com/2008/04/east-london-line-phase-2.html

A service train could in theory go from Highbury & Islington (ELL platforms) via Dalston and Surrey Quays to Clapham Junction, reverse and go round the WLL and NLL to Highbury (NLL platform). There is a single track connection from the NLL to one of the terminating ELL platforms at Highbury, but only for stock movements, not passenger workings.

At Clapham, the SLL will dive under the main lines to join the WLL at Ludgate Junction before terminating at a new Platform 2A (possibly) which will be built out over the track at the end of the part of Platform 2 where WLL trains terminate. Details of this part of the route are shown in a wonderful 1912 map here en.wikipedia.org/wiki/Clapham_Junction_railway_station (scroll down to the bottom, past the impossible schematic map, to a little thumbnail on the right that you can click on and expand).

The planned service has every other train from Clapham on the WLL continuing via Willesden Junction to Stratford, although TfL has aspirations to run them all (at least in the peaks) that way, but it will depend on the NLL resignalling, and getting/agreeing paths for freight in what will then be a 7½ minute frequency between Willesden and Stratford.

On the SLL/ELL, the service from Clapham will terminate at Dalston Junction. Trains from Highbury via the ELL will run a 7½ minute service alternately to Crystal Palace and West Croydon.

The list of OSI can be found here, you are allowed 20 minutes for the change.