Osei began his career at his homeland club Kessben fc now Medeama sporting club, On 28 February 2008 Ransford Osei was set to join Polish club Legia Warszawa on a six-month loan contract which gave the Polish club the option to buy the player during that time. Due to a work permit issue, the transfer was blocked by FIFA. Osei stayed in Kessben.
In 2008, he joined Israel based side Maccabi Haifa on loan. He subsequently signed a three-year contract of employment.
In July 2009, Osei joined Dutch side FC Twente on a year-long loan deal.
In August 2010, Granada CF announced that they had signed Osei on loan, and that he would join up with their second team Granada B.
After a short trial period, South African club Bloemfontein Celtic gave Ransford Osei a six-month contract in January 2012.
In July 2012, Osei signed for twenty-two-time champions of Ghana, and two times champions of Africa, Asante Kotoko. In September 2013, Osei turned down a contract renewal by Asante Kotoko, before being left out of Asante Kotoko's 2014 CAF Champions League's squad in the December.
In January 2015, Osei signed for South African Premier Soccer League side Polokwane City on a three-year contract. After managing just three substitute appearances due to injury, Osei was released by Polokwane City prior to the start of the 2015–16 season.
After going on trial with Kazakhstan Premier League side FC Shakhter Karagandy at the beginning of February, Osei played for RoPS in their Finnish League Cup match against PS Kemi Kings on 16 February. 5 days later, 21 February, Osei signed a two-year contract with RoPS.
A Striker described by FIFA.com as "an opportunistic goal machine" with superb pace and movement, young free-scoring sensation Osei was recognized by World Soccer Magazine as one of the 50 Most Exciting Teenagers on the Planet in their November 2007 Issue.
When it comes to designing the perfect striker, he has one particular model in mind. "I'd love to have Patrick Kluivert's heading ability, the speed and ball control of Samuel Eto'o and the finishing skills of Thierry Henry. I've got a few videos of him that I watch very closely". "Whenever I miss chances I can't sleep at night," Osei, the adidas Silver Shoe winner at the 2007 U-17 World Cup, tells FIFA.com. "I go through them over and over again in my mind, replaying them until I find the exact movement I should have made out on the pitch. When I'm sure I've learned my lesson I relax a little bit, but even then I know I'm not going to sleep that well.".
Second place, Silver (Golden Boot)
On August 20 he found the net twice in the 12 and 44th minute against Trinidad and Tobago in Ghana's 4–1 win during their first group game. Osei went on to score two more goals against Germany and Colombia, to take Ghana into the last sixteen of the FIFA U-17 World Cup, where they met and defeated favorites Brazil despite playing the entire second half with only ten men
On September 1, Osei set up the first goal and scored the second as the Ghanaians beat Peru 2–0 to take them into the U-17 semi-final match against Spain.
Osei finished the U-17 World Cup with the Adidas Silver Shoe as the competitions second highest goalscorer, coming second by one goal to Macauley Chrisantus of Nigeria. He scored his sixth and final goal of the Tournament came in the third place play-off in the tournament as Ghana lost 2–1 to Germany.
Osei scored in his first game against Uzbekistan in the 2009 FIFA U-20 World Cup one goal and played 90 minutes. He scored also another goal in his second game against England national under-20 football team and played 85 minutes.
On November 13, 2007, three weeks before his 17th birthday, he received his first senior International call-up from Ghana coach Claude Le Roy for a FIFA International friendly match against Togo at the Ohene Djan Stadium in Accra, Ghana, on November 18, 2007 after scoring a superb hat-trick against Togo's U17 team in a Junior International friendly at the same venue on Sunday November 11, 2007. He was selected among the nations 2010 Africa Cup of Nations squad even though he didn't appear in any of the matches. Ghana won silver in that tournament.As of match played 9 May 2015
Statistics accurate as of match played 11 August 2010Maccabi Haifa
Israeli Premier League (1): 2008–09
Ghana Premier League (1): 2012–13
Ghana Super Cup (1): 2012–13
FIFA U-20 World Cup (1): 2009
African Youth Championship (1): 2009
2009 African Youth Championship – (Golden Boot)
2007 African Under-17 Championship – (Golden Boot)
2007 FIFA U-17 World Cup – (Silver Boot)