M'darlin - you have come to the right place.
William John Oates, farmer, of Roswick, St Keverne son of Christopher Oates married Elizabeth Trewhella Rowe 16th February 1905 Helston UMFC. (You will find the entry in the Helston UMFC Registers - this being a Methodist Circuit)
Elizabeth Trewhella Rowe was born about 1872 (not found her baptism as yet) in Crowan - daughter of Alexander Stuart Rowe and Elizabeth Sally (nee TREWHELA).
I can supply further information on Elizabeth Sally Trewhela if, and when, required - although there may be a delay due to my ongoing move and settlement.
Also - I had not pursued this family so was not aware of the children you have listed. (Thank you for that)
Now, here is what I have found about William John Oates (before I go and pack more boxes and get them moved).
1861 Census - Roswick, St Keverne
Christopher Oates, head, mar, 52, farmer of 65 acres, St Keverne
Phillippa do., wife, mar, 46, St Keverne
Grace E do., daur, unm, 22, farmer's daur., St Keverne
Phillippa do., daur, unm, 17, farmer's daur., St Keverne
James do., son, unm, 15, farmer's son, St Keverne
Ursula J do., daur, 11, scholar, St Keverne
William J do., son, 7, scholar, St Keverne
Elizabeth Ellen do., daur, 6, scholar, St Keverne
Christopher do., son, 4, scholar, St Keverne
William Williams, nephew, unm., 30, carter, St Keverne
William Oates, nephew, unm., 17, carter, St Keverne
1871 Census - Roswick, St Keverne
Christopher Oates, head, mar, 64, farmer of 60 acres, St Keverne
Phillippa do., wife, mar, 56, St Keverne
Ursula J do., daur, unm, 21, St Keverne
William J do., son, unm, 17, St Keverne
I can pursue other Census records later but it certainly appears this is your man.
It appears Christopher and Phillippa were not married in St Keverne - however I have found the following baptisms at St Keverne:-
Christopher Oats bp. 12th October 1806 s/o William and Millicent
24th December 1837 Grace Exelby d/o Christopher & Phillippa Oates of Traboe, Publican
7th July 1839 Milicent d/o Christopher & Phillippa Oates of Traboe, Innkeeper
6th June 1841 Ann d/o Christopher & Phillippa Oates of Traboe, Innkeeper
19th November 1843 Phillippa d/o Christopher & Phillipa Oates of Traboe, Innkeeper
13th July 1845 James s/o Christopher & Thomasine Oates of Traboe, Publican
31st July 1848 Ursula d/o Christopher & Phillippa Oats of Roswick, farmer (Received into Church 28th August by J Lane)
10th July 1853 William John s/o Christopher & Phillippa Oates of Roswick, farmer (Private Baptism)
8th April 1855 Elizabeth Ellen d/o Christopher & Phillippa Oates of Rosewick, farmer
Have not looked at burials yet but did notice a marriage for Grace Exelby Oates in 1863 (but only noticed it in passing).
Hope this is of some help and I will continue with it further once I am all set up again.
Ian