> If you're trying to say that when those two denote /o:/ it is a different /o:/, you are laughably wrong.

There's literally a different sound, which is why the difference in kana exists. Disagree if you like -- as I said, it's subtle -- but I don't know why you feel the need to be insulting about it. Writing an inaccurate non-kana symbol for the two sounds is no more an argument than saying that the sounds are identical because they share a common romanization.

There are some words where you can more clearly hear the difference than others. Consider, for example, the pronunciation of 紅茶, vs your example of 氷. It's not wrong to pronounce the former as a long o, but you can hear the difference when natives say it. Similarly, こういう is not said as こおいう, and 公園 is not こおえん.