Might be rehashing old ground for you, but...
In 1851 census, Oliver Douglas from 1841 census is down as born Yetholm Roxburghshire, in the Scottish Lowlands on the border with England.
Clibrig, Farr, Sutherland,
James Douglas, 45, head, Shepherd, b. Huetina (?) Northumberland,
Cathrine Douglas, 37, wife, b.Assynt Sutherland,
Elisa Douglas, 12, dau, b. Farr
Alexander Douglas, 10, son, b. Farr,
Oliver Douglas, 10, dau, b. Farr,
Catherine Douglas, 9, dau, b. Farr,
Thomas Douglas, 7, son, b. Farr,
Jean Douglas, 4, dau, b. Farr
Oliver Douglas, 80, mother, Former Shepherds wife, b. Yetholm Roxburgh,
Some baptisms on the IGI, all at Yetholm Roxburgh, children of Thomas Douglass and Oliver Young.
William Dougless, 7 Dec 1798,
John Douglass, 26 Jul 1800,
Thomas Douglass, 4 Oct 1802,
James Douglass, b 10 May 1805, bap 21 May 1805.
The baptism of James Douglas.
May 10. Thomas Douglas, Hind in Shotton, and his spouse Oliver Young, had a son born and baptized 21 May, James.
Shotton is over the border in Kirknewton Parish in England.
Scottish deaths certs were very detailed and some kind person has attached Olivers to a family tree on Ancestry.
1855, Deaths in Parish of Farr.
Oliver Douglas, female, 86 years. Born Yetholm. 36 years in Strathaven (?). Father - John Young, Woollen Weaver, deceased. Mother - Margaret Young, Maiden name Oliver, deceased. Wife of Thomas Douglas, Shepherd, deceased. Children - William, 56, John 55, Thomas 53, James 51, Walter 47, Andrew - deceased in ?, Oliver - deceased in ?, Elizabeth - deceased in ?. Died January 28th 1855 at 8am at Authituird, Parish of Farr, of Old Age. Informant - John Douglas - son.
So it doesn't look like the Thomas, father of William born in Lowick in 1816, is the same Thomas that William is living with in 1841.
And Thomas and Olivers son Thomas went to Australia as well.
Nepaul Passenger List. Arriving New South Wales, 16 Mar 1855.
Thomas Douglas, 49, Shepherd, b. Kirknewton, Northumberland. Parents - Thomas, and Oliver Young. Father dead, mother living in Sutherlandshire. No relations in colony.
Mary Douglas, 45, b. Branton, Northumberland. Parents Anthony and Jane Paterson. Father at Caithness, mother dead. Relations in colony - Michael and Thomas Paterson, 2 brothers living at Gunning.
And 9 children, all born Sutherland.
Maybe needs a bit more trawl of the Lowick Presbyterian records for any siblings for William.
Alan.