Speaking from the states, alot of people are talking as if Oasis is/was an underground thing. They defintley had a heaping amount of success in the mid 1990's, and in Cleveland, I still hear Wonderwall on the radio sometimes.
Of course in the U.K, they were the definitive band of the time, probably much like the Stone Roses, and good luck finding someone in the states having any idea who Sally Cinnamon is.
The cultural impact that Coldplay had here seemed to be at a pinnacle in 2003, when interest and momentum was steamrolling around the time of Clocks, but never have they been the "Seminal Rock Band" like Oasis was during their heyday in the UK
Record sales are one thing, but capturing an imagination is an other, and Oasis is still playing big venues here in North America, although Coldplay is most defintley bigger.