Oram, the 34-year-old former New Zealand allrounder, was Worcestershire's initial signing as second overseas player, alongside Sri Lankan Thilan Samaraweera, but he now needs hernia surgery and is unable to fulfil his commitments.
"I'm extremely disappointed to miss out on the opportunity to represent Worcestershire," Oram said. "I have never played county cricket before so I was really looking forward to the experience. Hopefully I can fulfil my ambition next season, but for this season I wish Steve Rhodes and Worcestershire all the best."
Russell, 25 from Jamaica, will join Worcestershire for the first nine Flt20 matches. He scores his runs at a strike-rate of 148.87, is a fine fielder and is currently playing for Delhi in the IPL. He was part of the West Indies side that won the World T20 in 2012.
"Having previously experienced some time at Worcestershire practicing with the squad I am very much looking forward to playing in their t20 competition," Russell said. "I have always wanted to play county cricket and I am excited about being involved in such a professional and friendly environment."
Director of cricket Steve Rhodes said Russell's signing had softened the blow of losing Oram. "Andre is a high class international allrounder. He has ability with the bat, ball and in the field and if he connects with the middle of his bat there are few grounds big enough. He will add firepower and excitement to our campaign."