Upvote:2
A quick search on travelinesoutheast.org.uk gives you several options:

    Depart   Arrive   Changes   Max journey time   Trip Summary
1.  0643     0817     1         01:34              walk, coach, bus, walk
2.  0658     0852     2         01:54              walk, bus, walk, train, train, walk
3.  0728     0922     2         01:54              walk, bus, train, walk, bus, walk
4.  0752     0929     1         01:37              walk, coach, walk, train, walk
5.  0822     0955     1         01:33              walk, coach, walk, train, walk
6.  0828     0955     2         01:27              walk, bus, bus, walk, train, walk
7.  0828     1000     1         01:32              walk, bus, bus, walk
8.  0936     1105     1         01:29              walk, bus, walk, train, walk
9.  0956     1131     1         01:35              walk, bus, walk, train, walk
(none of which are particularly pleasing). The one that gets you there for about the right time is:

Option 2
0658: Oxford (Oxon)
walk: From the centre of Oxford (Oxon) walk for about 2 mins0700: Oxford City Centre, Turl Street, Stop L1 (on High Street) [SMS: oxfgapmd]
bus: Take Arriva the Shires & Essex Bus 280
towards Aylesbury, Bus Station
and get off at0754: Haddenham, adj Railway Station (on Station Forecourt)
walk: Walk about 4 mins to0811: Haddenham & Thame Parkway Rail Station
train: Take Chiltern Railways Train
towards London Marylebone Rail Station
and get off at0818: Princes Risborough Rail Station
walk: Then transfer to0838: Princes Risborough Rail Station
train: Take Chiltern Railways Train
towards London Marylebone Rail Station
and get off at0843: Saunderton Rail Staion
walk: Then walk for about 9 mins to the centre of Saunderton0852: Saunderton
So a bus and two trains and a fair bit of walking.
Personally, for that kind of distance Iβd order a cab. Thatβd be more expensive but much, much more convenient and faster! Thereβll be plenty outside of Oxford train station at that time of the morning. Give yourself plenty of time to allow for the rush hour traffic, which is not pleasant in Oxford.