I love Green Day and I love Op Ivy, I don't see why people have to debate over it. Obviously Green Day looked up to Op Ivy a great amount, hence the cover of this song. Everyone has an opinion on the bands, but there's no need to fight about it.
Back in the late eightees when Billie Joe was a teenager, has was a huge fan of Operation Ivy. They were another bay area band and a big influence to their early music. So, he did a cover of Knowledge in their first album in 1990, but it's not really a cover because it's a slow version.