I've never heard of the /o:/ of おう and おお being different. I've never seen a small child, or foreign speaker, being corrected in this matter; i.e that they are using the wrong /o:/ for the word and should make it sound like this instead.

This is literally not a thing that exists outside of some foreigners' imaginations. You will sooner hear a difference from $1000 speaker cables before you hear this, and it will only be if you are the one who paid.

You may be letting by pitch accent deceive you. In words that contain /o:/ it's possible for that to be a pitch boundary so that pitch rises during the /o:/ and that can contrast against another /o:/ word where that doesn't happen.

The 頬 word in Japanese is "kinda funny" in that it has a ほお variant and a ほほ variant. It has always stood out in my mind as peculiar. I'd swear I've heard an in-between "ほ・お" that sound somewhat reminiscent of "uh oh", with a bit of a volume dip or little stop that makes it sound like two /o/ vowels. It could be that the speaker intends ほほ, but the second /h/ sound is not articulated clearly. It may even be that the ほほ spelling was invented to try to represent this situation (which is a wild guess, based on zero research). In any case, the situation with that cheeky little word doesn't establish anything general about おお/こお/そお/とお...

I've been fooled by my imagination. For instance, many years ago I thought I would swear that I heard the object marker を sound like "WO" in some songs; i.e. exactly how it typed in romaji-based input methods, because it belongs to the わ group. Like "kimi-o" sounding like "kimi-wo". Today I'm convinced it is just a kind of 空耳 (soramimi). Or the artifact of /i/ followed by /o/ without interruption, becoming a dipthong that passes through /u/: it may be real, but unintentional. It's one of those things that if you convince yourself is real, you will tend to interpret what you are hearing in favor of that.

E.g. in Moriama Naotarō's "Kisetsu no mado de" (季節の窓で), right in the first verse. https://www.youtube.com/watch?v=8FjvNqg3034

That's actually a good example because there are so many covers of that, you can see whether you hear the "whoopy wo" from differnt speakers.

There is a similar situation in the pronunication o 千円. There is a ghost "ye" that appears to the foreign ear. To the point that we have developed the exonym "yen" for the Japanese currency!!! The reality is more like that the /n/ is nasalized, similarly to what happens when it is followed by /g/. https://www.youtube.com/watch?v=5ONt6a1o-hg

OK, finally, let's crack open the a 1998 edition of the the NHK日本語撥音辞典. On pages 832-833, we have all the /ho:/ words, with their pronunications including pitch accents:

ホー with falling accent after ホ: 方、砲、鵬、朴

And, our cheeky word 頬 gets a separate entry here due to its pronunications ホー and ほほ。Both have a falling pitch after the leading ほ, like 方. No difference is noted.

ホー with pitch rising at the "o": 法、報

So of course if you compare someone saying 法律 vs 頬, there will be a difference. But a lot of longer ほお words have the same rising pitch like 法. 法律 (ほうりつ） vs 放り出す （ほおりだす）is the same.

Fairly intuitively, 頬張る（ほおばる）has rising pitch at the お、in spite of 頬 by itself exhibiting falling pitch.