Apologies for coming late to the party, so to speak, but I have been tracking down the forebears of my ancestor Mary Oxlade wife of William Crawford, saw this thread and thought I could contribute.
It seems to me that the Oxlad references belong to a different family - the 1788 marriage of William Oxlad does not match either William Oxlade b 1743 (then married to Eleanor Waller) or his son William Oxlade b 1768 (William Oxlad was already a widower according to marriage record). Likewise the baptismal records from Mitcham are almost certainly from other branches of the Oxlade family (there are other records for Oxlades in Mitcham dating back 50 years or so). Also, if William Oxlad is unrelated to William Oxlade, bookbinder, then the 1803 burial for William Oxlad is almost certainly not that of William Oxlade, bookbinder. Furthermore, there is no reliable record for the death of his son, William, born 1768 - although there is a burial in Dulwich College for 1769, this is more likely to be William, son of Richard Oxlade, baptised in Dulwich 1768.
So, William Oxlade of Portsea could be either William, b 1743 or his son. It is not necessary to assume that John Oxlade was printing in his father's name. Other than his marriage to Mary Ann Terry in 1813, there may be little to connect John Oxlade to Portsea. One point of interest, though, is that John and Mary Ann were living in Peerless Row when their children, Henry, John Thomas and Sarah Elizabeth, were baptised. Peerless Row was also the address of William and Mary Crawford.
I am, however, a little perplexed by John Oxlade presenting himself as a bachelor at his wedding to Mary Ann. In 1798, John Oxlade, bookbinder, and his wife Ann, baptised a daughter, Ann, a couple of months before his arrest. So why present as a bachelor ( unless it is a simple mistake)?