You have me more than a little confused with your query as well!!
I presume, but am not sure, that this question is about a marriage for the son?
IN the first quote above you indicate that you think the wife of the elder Philip was Jane Stone but in this quite you are asking if he married Jane Oats!
And then you say that you think he remarried to Jane Martyns in 1800. But which Philip are you talking about now?
You should find answers to most of these in the OPC records but here are a couple of things I have found.
A Philip was baptised to Philip and Jane Harvey at St Just in 1758. BUT this record also indicates that the father was DECEASED.
Philip son of Philip Harvey (deceased) and of Jane his wife baptised 11th June 1858 St JustSt Just records show a burial for a Philip Harvey at St Just on March 10th 1758. It is likely this is the Philip Harvey baptised at St Just in 1733 to Philip Harvey.
Now this first Philip appears to have married Cheston Champen at St Just in 1730 (record available on OPC).
Cheston wife of Philip Harvey was buried 16th November 1737 at St Just and then on 6th April 1838 at St Just there is a marriage for a Philip Harvy to Jane Stone. Although the record does not state he was widowed I should think this is possibly the husband of Cheston.
Now at this point I admit it does start to become a little confusing. There is no burial at St Just for a Philip Harvey between 1733 and 1757 so as I said at the start the Philip Harvey whose son was baptised in 1758 would seem to be probably the Philip baptised in 1733.
The confusion comes because there appears to be no other marriage for a Philip Harvey to anyone named Jane which would account for the 1758 family.
However, there is a marriage at St Just on 12th July 1753 for a Philip Harvey to Alice Pelew! I am beginning to think that there may be an error in the St Just register with Jane being incorrectly entered as the mother for Philip Harvey in 1758.
Here is why I suspect that possibility. The St Just Parish Register shows the following baptism:-
Frances daughter of Philip Harvey and Alice his wife was baptised March 27th 1757The Bishops Transcripts for that same event apparently show the mother of Frances as JANE!
And then in 1758 we have the baptism of Philip to Philip and Jane and although the BTs apparently also show the mother as Jane for this one I still suspect an error. The BTs are copied from the register but errors do occur in both.
Now the Philip baptised in 1758 - there are two possible marriages for this man, one in 1775 to Jane Oats and one in 1781 to Margaret Roberts. As he would have been only 17 in 1775 I would think it more likely this Philip married Margaret Oats.
You are going to have to look at all these records for yourself and perhaps search for some Wills or other information that might help and then make your own decisions about the families.
It is perhaps a little easier to begin a little later and work backwards from some information that is more easily verified.
But there is one thing I can confirm and that is about the 1800 marriage of Philip Harvey, widower, and Jane Martyns, widow.
I have checked the entries in the St Just register and the signiatures show that the Philip Harvey who married Margaret Roberts in 1781 is the same man who married Jane Martyns in 1800.
Also interesting to note is that the same Phillip Harvey was a witness to the marriage of John Martins and Jane Hattam!
CT
You have me more than a little confused with your query as well!! 
