The O Rings that would have kept the Challenger crew alive were banned because they contained some chemical that the extremists at the EPA didn’t like.
It sounds like you're confusing the Challenger's SRB O-rings with Columbia's insulation and ice impact problems. The latter was the one where NASA had a waiver and could have used the older formula (with non-EPA-approved chemicals), but chose to use the "green" formula.
I don't recall any discussion of the o-ring material being compromised by EPA meddling (though of course, I wouldn't really be surprised at such a revelation).
That could well be. My memory of it all is vague and it’s likely I could be confusing the two events.