I don't know if you have access to the Northmoor PRs, but they actually make quite interesting reading!
I found these four baptisms, which particularly caught my eye because of the variant surname and the father's forename:
11 APR 1654 - DAUSTON Thomas, s. Humfry and Margreet (born 11 MAR)
16 OCT 1657 - DAUSTON John, s. Humphry and Margreat (born 11 SEP)
06 JUN 1660 - DASTON Francees, d. Humphry and Margret (born 30 APR)
18 AUG 1663 - DASTON Martha, d. Humphry and Margreat (born 11 JUL)
Although unprovable, I think it very likely that your Humphry could be the son of Humphry and Margret, particularly when you see how the surname evolves as the parish register continues (parents' names in brackets):
1665 - DASTON (Richard and Anne)
1666 - DASTON (Richard and Anne)
1667 - DASTON (Richard and Anne)
1671 - DAWSON (Thomas and Anne)
1671 - DAWSON (Richard and Anne)
1676 - DASON (Thomas and Anne)
1680 - DASON (Thomas and Anne)
1682 - DASON (Thomas and Anne)
1684 - DAWSON (Thomas and Anne)
According to the OFHS's Oxfordshire Marriage Index (1538-1837) your Humphry married a Joan (not Jane) and I'm assuming that these are their children baptised at Northmoor. Again, the evolution of the surname is interesting (and also the variations on the parents' names, typed as found on the transcript!):
1677 - DAUSTON (Umfry and Jone)
1681 - DASON (Umfri and John)
1683 - DAUSON (Humphry and Joan)
1686 - DAWSON (Humphry and Joan)
Hope this sheds a little light on the matter!
Pennie