Tom Greeves writes (personal communication to Mark Beeson, 2/1/2020):
My two informants about Jan Oo were John Hamlyn (born 1883) and Annie Sleep (born 1892), both born and bred in the Postbridge area.
On 9 March 1982 Annie Sleep told me of ‘Jan Oo’s House’ ‘on the west side of the Dart, beyond Hartland Tor.’ This is beside the East Dart at SX 63908102. She had mentioned ‘Jan Oo’s House’ to me before this date, but I can’t find a record of it, despite searching through 50 years’ of notebooks!
On 12 November 1982 John Hamlyn told me of a house north of Stannon Tor at SX 64758143 which he knew as ‘Jan Oo’s House’ and an enclosure (SX 64888161), a short distance north-east of it, as ‘Jan Oo’s Garden’. He said that the House was occupied by a man who previously lived in the one by the East Dart [which Annie Sleep had mentioned as ‘Jan Oo’s House’] but who got ‘washed out’. He thought this might have been due to the Powder Mills leat bursting, but I would think it was more likely due to an exceptional flood on the East Dart. The Powder Mills leat was not constructed until the 1840s, and I suspect both buildings date to the 18th century. John Hamlyn didn’t know why the East Dart building was built but he thought it might have something to do with the creation of Broadun Newtake. This is interesting as some of these Postbridge newtakes certainly date back to the 17th century but an 18th century date for the newtake wall above the building is quite possible.
John Hamlyn went on to say that ‘Jan Oo’ was really ‘Jan Wood’. In Dartmoor dialect ‘Wood’ becomes ‘ ‘ood’ or ‘hood’ and thence ‘oo’, as in the O brook, sometimes written Oo Brook or Wo Brook (William Crossing’s Guide to Dartmoor).
So I think we can conclude that tales of Jan Oo were current in the Postbridge area in the late 19th century, if not earlier, and associated with two specific buildings. Quite how they relate to the Rowbrook tale is yet to be resolved.
Given that ‘O Brook’ is an unusual and striking name, close to the Rowbrook location for Jan Coo/Oo, I would think it a strong possibility that it might have been named after Jan Coo/Oo as you suggest. Otherwise one would have to conclude that the name had a prosaic origin, entirely based on its ‘woody’ character. But it is only significantly wooded now between Week Ford and Saddle Bridge so unless the name relates to a time when it was wooded much further upstream (medieval or prehistoric times), it would seem not improbable that its name may well have a link with the Dart itself and its woodland spirit.