John Joseph William Molesworth Oxley (1784 – 25 May 1828) was an English explorer and surveyor of Australia in the early period of British colonisation. He served as Surveyor General of New South Wales and is perhaps best known for his exploration of the Tweed River and the Brisbane River in what is now the state of Queensland.

John Oxley was born in 1784 at Kirkham Abbey near Westow in Yorkshire, England, and baptised at Bulmer in St Martin's Church on 6 July 1784. He was the eldest of eight children of John and Arabella Oxley and was a Protestant.

In 1799 (aged 15), he entered the Royal Navy as a midshipman on the Venerable (1784). He travelled to Australia in October 1802 as master's mate of the naval vessel Buffalo, which carried out coastal surveying (including the survey of Western Port), and this first stay in the Colonies would last for five years. In 1805, Oxley became acting lieutenant of the Buffalo and traveled to Van Diemen's Land the following year in charge of the Estramina.^{[citation needed]}

He returned to England in 1807 and from there he was appointed first lieutenant of HMS Porpoise, a British sloop of war that was stationed at NSW. To take up this appointment he sailed out again to NSW on the Speke as part of the Transport Board. He arrived in November 1808 with £800 of freight transport. In 1809 Porpoise visited Van Diemen's Land, carrying as a passenger Governor William Bligh, who had been deposed in the Rum Rebellion. When Bligh was deposed, Oxley denied he supported John Macarthur but his letters showed that he was close to him. In 1810, Oxley returned to England. During this period, Oxley sought the positions of Naval Officer and Surveyor-General. He retired from the Navy in 1811 and was briefly in an engagement to Elizabeth Macarthur the following year.

In 1812, Oxley travelled to Sydney as Surveyor-General of the Minstrel. Oxley's appointment was at the time of Lachlan Macquarie's Governorship. Macquarie encouraged exploration – he had sent George Evans to confirm the exploratory work of Wentworth, Blaxland and Lawson over the Blue Mountains, instigated the building of the road over the Blue Mountains in 1814–1815, and had travelled to Bathurst immediately William Cox had completed it. From there he had sent George Evans on an expedition of exploration up the Lachlan River in May 1815.^{[citation needed]}

Now Macquarie wanted the Lachlan and Macquarie River explored thoroughly. Opening up of the new lands over the mountains had created enthusiasm for further discoveries about them and the Macquarie River. Mysteriously, the Macquarie and the Lachlan flowed westwards to the interior of the country and not easterly towards the coastline. Successively, in 1817 and 1818 Governor Macquarie appointed John Oxley in charge of two expeditions to investigate these rivers.

On the 1817 Lachlan expedition, Oxley was to come across marshy country and conclude this inland area was uninhabitable. If he had pressed on for two more days he would have reached the Murrumbidgee River. Oxley reported that, in his opinion, the Lachlan flowed into an extensive series of swamps, "which were, perhaps, the margin of a great inland sea." Similarly, the Macquarie expedition the following year in 1818 came to a halt on that river at the Macquarie Marshes in a good season for the marshes as the Macquarie was in flood replenishing these vital wetlands. Oxley tried hard to proceed through them but couldn't do so. He returned to the encampment of the rest of his party now convinced that these westward flowing rivers terminated in an inland sea, and he had been on the swampy edge of it.^{[citation needed]}

Through Oxley, the theory of the Australian inland sea was fed and perpetuated.^{[citation needed]}