In essence:-
There’s a DOO geographical area - essentially Paddington to Bedwyn and Banbury, plus Thames branches.
IET-operated services may run DOO(P) if their whole, originally timetabled journey starts AND ends within that geographical area.
Such trains must be dispatched by station staff at stations; so that introduces requirements for guards for services that stop at unstaffed stations a la
@FGW_DID ‘s Radley example above.
Ordinarily the only staffed stations are Padd, Slough, Maidenhead, Reading, Didcot and Banbury. Twyford is sometimes staffed.
10 cars aren’t permitted DOO(P).
Any IET-operated service that goes outside the DOO area for any part of its journey; must have a guard for its entire journey.
The same rules do NOT apply to 16x, 230 or 387 - they may run DOO at any time within the DOO area; whether part-journey or not.
No other GWR traction is currently approved for DOO(P) operation at present.
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So Paddington <> Oxford services are likely DOO(P)
Paddington <> Bedwyn services are normally DOO(P) to/from Reading then guard beyond.