After discussion about Cannon Street - Bank being missing from a
possibly incomplete list of OSIs, I asked TfL, who eventually replied
stating that it is an OSI.

They even gave specific interchange times, 15 minutes between Cannon
Street Underground and Bank, and 40 minutes between Cannon Street NR
and Bank (I'd have thought that would vary with direction, but it's
what they said).

Then today, a Saturday, I was charged £3.50 for a journey from
Deptford to Holborn, ie £1.70 plus £1.80, instead of the through fare
of £2.80.

So they confirmed that it is an OSI after all, and then I found that
it isn't an OSI after all after all.