Batsman Ian Bell has also been added to the England XI who played Warwickshire last week while left-arm spinner Monty Panesar wins selection for the match starting on Wednesday ahead of Yorkshire leg-spinner Adil Rashid.
Harmison displayed genuine pace and disconcerting bounce for the England Lions against Australia in the drawn match that finished on Saturday, dismissing exciting new opener Phillip Hughes cheaply in both innings and taking six wickets overall.
However, he paid the price for his frustrating inconsistency in test cricket while Onions has reaped the reward of a fine first series against West Indies this year, including five for 38 figures on debut.
"Graham Onions was picked on what he has done in his last two test matches and his consistency in county cricket as well," chief selector Geoff Miller told Skysports.
"It was encouraging to see how Steve Harmison bowled but it's not a sprint these Ashes, it's a long marathon and there's a lot of cricket to be played."
Bell has been included as the extra batsman while Panesar's respectable test record gives him the edge over the inexperienced Rashid if England decide to play two spinners.
Cardiff will be staging a test for the first time and the England team management will examine the pitch closely before making a decision on the final XI.
"We have got the options there with spin, we could play two spinners and three seamers. We'll wait to see what the pitch produces," Miller said.
The batting lineup looks settled with wicketkeeper Matt Prior likely to bat at six followed by Andrew Flintoff who is returning after yet another injury break.
James Anderson will head the pace attack assisted by Stuart Broad, Flintoff and Onions if England opt for Graeme Swann as the only spinner.
