Oh. This knocks me sideways. However, I've been following the growth of a substantial Oxlade family website that would appear to suggest or confirm the following details - that add up...
JO's first family consised of Richard, b. Mitcham, c. 1793; d. Camberwell c. 1841 George, b. Mitcham 1795 and dyingin the same year
Saqrah, b. Mitcham 1796 and dying in 1812
Ann, b. Mitcham 1797 (no date of death so far)
Eliaberh, b. Mitcham 1798; d. 1812
JO himself, it seems, worked as a shoemaker in Finsbury after removal from Portsea up until 1841 and died in 1850.
In between, his family with Mary Ann Oxlade was as follows:
Henry, b. Finsbury c. 1820
John Thomas, b. 1824
Sarah Elizabeth, b. 1825
Mary Ann, b. 1829
Given the names Sarah Elizabeth and Matry Ann, there would seem to be a stgrong connection. In the end, then, your findings may have been of a different family.
Onwards and sideways
roly