Born in Wakefield, West Yorkshire, Oliver (Oli) Johnson started his career in the ninth tier of English football with Nostell Miners Welfare, scoring nine goals during the 2007–08 season. He impressed on trial with Stockport County and joined them on 17 October 2008, signing a contract until the end of the season and made his debut as part of a 2–1 loss against Colchester United. He made 15 starts and 28 substitute appearances in his 15months at Stockport, scoring six goals.

Whilst with Stockport, he was linked in May 2009 with a move to Fulham

Johnson signed for Norwich City on a two-and-a-half year contract for an undisclosed fee on 8th January 2010 and made his debut coming on as a late substitute in Norwich's 3–1 victory over Exeter City the following day. He scored his first City goal in the 5-0 win over Colchester the following week. He finished the season with 4 goals in 17 appearances as City won the League One title.

On 14th January 2011, Oli joined Yeovil on a month's loan as he had made just 6 appearances, all from the bench. for Norwich during the 2010/11 season. This loan was extended to the end of the season and Oli played 17 games, scoring three times

Not selected as part of Norwich's 25 man Premier League squad for the 2011/12 campaign. Oli was allowed to join Scunthorpe on trial in early September 2011. He returned to City shortly after when the two clubs could not agree over who would pay Oli's wages. He rejoined Yeovil on a three month loan on 26th September 2011

He returned to Norwich in January 2012 having made 9 appearances for Yeovil and without scoring a goal. His Norwich contract was cancelled by mutual consent on 9th January 2012. He then trialled with Carlisle. They had hoped to trial Johnson in a friendly against Doncaster that was called off due to a frozen pitchHe told the Wakefield Express: "It has gone well at Carlisle and they seem pretty happy with me. I am just waiting to see if they are going to offer me a deal and I am hopeful that I might find out by the end of the week. Any deal has got to be right for the club and me." Barnsley were also said to be interested

Ultimately Oli opted for Oxford United, signing for them on 20th January 2012

Oli was released by Oxford at the end of the 2011/12 season, having made 17 appearances and scoring three goals.

Oli returned to Yorkshire in June 2012 signing for York City on 26th June 2012

Johnson left York at the end of February / beginning of March 2013 by mutual consent having made just seven appearances due to stomach and groin injuies.. He then joined Guiseley on 16th January 2014.