Sorry I'm getting into this a bit late

John Oxlade and Sarah Sheldrick marry in 1794 - Aldgate - witness is William Oxlade
Baptisms - no mother's name given, nor occupation:
Frances Susannah - 1793 (baptised 1802) - Whitechapel - so born before they married
William Thomas - 1795 - Whitechapel
Richard -
1793 - Mitcham - possibly buried 1805

George - infant son of John - buried 1795
Sarah - 1796 - Mitcham
Elizabeth - 1798 - Mitcham
Henry - 1820 - Islington - John is a Shoemaker and married to Mary Ann
John Thomas - 1825 - Islington - John is a Book binder and married to Mary Ann
Sarah Elizabeth - 1828 - Islington - John is a Bookseller and married to Mary Ann
I can't readily see a Mary Ann in 1829. I haven't attempted to look for him in Portsea.
My view is that you have more than one John Oxlade here.