Question: can you see oahu from the big island?
Answer: Of course from Oahu you can see Maui (back behind Molokai and Lanai) on clear days too, from the Kokohead / Sandy Beach / Makapuu area. However, I say that the mountains on Kauai are shorter than those on Oahu and Maui, it being an older island, so it can't be seen from as far a distance.