I'm not pre-privatisation (joined in 2006), and I work for a bus
company other than East Thames.

Never actually used an overground service, however whenever I tap my
oyster on a tube ticket machine it is displayed as something like:

O/E Bus Operator
Zones 1-9 + Watford Junction

So I naturally assume that means I can use it there! Just thought that
if I had been given those PAYG services that the pass might be valid
wherever PAYG is if you get my drift. Maybe best to contaft TfL staff
travel themselves.