Date of birth is listed as circa 1806 from unconfirmed information (blue hand-written document of unknown source) collected by Brian Oliver of nzolivers.com (b.1950) on his April 2008 visit to Devon.

However, Dax Eric Clayton Lloyd, great-grandson of Jessie Bertha Oliver b.1876, says in an email of 12 July 2006 John was Christened 11 April 1802 in Holsworthy, Devon, England:

From www.familysearch.org
Christening:11 APR 1802 Holsworthy, Devon, England
Parents:
Father: JOHN OLIVER
Mother: GRACE
Messages:
Extracted birth or christening record for the
locality listed in the record. The source records are
usually arranged chronologically by the birth or
christening date.
Source Information:
Batch No.: C051131
Dates: 1563 - 1837
Source Call No.: 917146
Type: Film
Printout Call No.: 933430
Type: Film
Sheet: 00

A year of birth of 1802 is supported by the 1851 Census.

This date of birth same as provided by Sarah Ferguson (nee Churton).

Entry found in 1851 England Census for Northcott Hamlet in Boyton Parish, County and Parliamentary division of Northern Division of Devon, for Boyton Mill
John Oliver, Head, Married, age 76, [Farmer of 35 Acres employing 3 Ag Lab, born Tetcott Devon, England [circa 1775]
Grace Oliver, wife, Married, age 73, born St Giles in the Heath Devon, England [circa 1778]
John Oliver, son, Married, age 49, born Holsworthy, Devon, England [circa 1802] [This John’s wife Mary is listed in the Northcott Villiage entry as a servant]
Thomas Bate, Servant, Unmarr, age 16, Ag Lab
Able Werrey, Servant, Unmarr, age 13, Ag Lab
John Martin, Servant, Unmarr, age 12

The 1861 Census lists a John Oliver living at Boyton House with wife Mary Oliver. John is listed as 59 years of age and born in Wharfinger, St Giles, Devon. This makes John born in 1802. His wife Mary is listed as 62 years of age and born in Ashwater, Devon.

In January 2012, Sherilyn Oliver (nee North) great-granddaughter of Francis Oliver b.1840, youngest child of James and Ruth Oliver, has supplied me with research of work she has done on the decendants of this John, b. 1802 and his wife Mary.

Sherilyn’s information agrees with the date and place of birth. She supplies the date of death quoted here.

This date of death of June 1879 provided by Sarah Ferguson (nee Churton).

From records online for Arch Deaconry of Totnes. Deanery & Parish of Holsworthy - Baptisms, as alerted by Katia and Tim Oliver In an email dated 4 January 2021.
Tim Oliver is a x3 great grandson of James and Ruth Oliver, his grandfather being Ivor James Clark Oliver (1907-1989).
Name: John Oliver
Event Type: Baptism
Event Date: 11 Apr 1802
Event Place: Holsworthy, Devon, England
Event Place (Original): Holsworthy, Devon, England
Sex: Male
Father's Name: John Oliver
Mother's Name: Grace Oliver
From Scans of original parish registers from Cornwall [including Devonshire] containing baptisms from 1538 to 1910 published on FamilySearch:
https://www.familysearch.org/ark:/61903/1:1:QGR8-H375
Note: Login to FamilySearch required

Date and place of Marriage, wife’s maiden name, from Devon Marriages and Banns records received from Katia, wife of Tim Grant Oliver, in the email dated 19 December 2022:
First name(s): John
Last name: Oliver
Marriage year: 1824
Marriage date: 03 Aug 1824
Spouse's first name(s): Mary
Spouse's last name: Sleeman
Place: Virginstow
Parish: Virginstow
Denomination: Anglican
County: Devon
Country: England

Details of burial from a link to an image in an email dated 20 January 2023 received from Katia, wife of Tim Grant Oliver.
Burials in the Parish of Boyton in the County of Cornwall in the Year 1872
Name: John Oliver
Abode: Boyton Bridge
When Buried: December 10 1872
Age: 71
By Whom the Ceremony was performed: Thomas Walters, Vicar [was Vicar 1870 to 1907]

Date of death of “7 December 1872”, died of “Paralysis 2 years Exchaustion Certified” from image of death register record attached to an email dated 8 July 2023 from Katia and Tim Oliver.