1925 Dict. Bootham Slang, Hoick,.. spit. Oick,.. to spit; abbreviated form of 'oickman'. Oickman,..labourer, shopkeeper, etc.; also a disparaging term. 1933 A. G. Macdonell England, their England vi. 95 Those privately educated oicks are a pretty grisly set of oicks. Grocers' sons and oicks and what not. 1935 'N. Blake' Question of Proof x. 189 Smithers is such an oick. 1940 M. Marples Public School Slang 31 Oik, hoik: very widely used and of some age; at Cheltenham (1897) it meant simply a working man, but at Christ's Hospital (1885) it implied someone who spoke Cockney, and at Bootham (1925) someone who spoke with a Yorkshire accent. 1940 M. Dickens Mariana iv. 109 The old Oik mentioned it over a couple of whiskeys. 1946 G. Hackforth-Jones Sixteen Bells 260 Come to think of it he must have been a bit of an oik when he worked at Bullingham & Messer. That crack about long hair was well merited. 1957 F. King Widow i. v. 63 He and Cooper had fought a battle with three 'oiks'—this was apparently school slang for the boys of the town. 1958 B. Goolden Ships of Youth vii. 162, I only need my cap on back to front to look the complete oick. 1959 W. Camp Ruling Passion xvi. 126 Who's that incredibly uncouth and oikish man? 1966 'K. Nicholson' Hook, Line & Sinker viii. 95 So glad you got here before the oicks. 1968 Melody Maker 30 Nov. 24/5 Old Stinks from the third stream said: 'I say you oik, the Beach Boys latest is fab gear.' 1975 Listener 16 Jan. 83/1 The rigmarole about the flat was patent set-dressing, just to impress us oiks. 1975 Times 7 Aug. 7/7 His sc. Oswald Mosley's angels, a gang of gullible and bloodthirsty oiks.. would come pretty far down the roster of hell's legions.
The Urban dictionary gives another two definitions:
Oik is an abbreviation for "only I know" and/or
British slang for "only income, kids" or "own income, kids". A person with children but no partner.