What’s the difference between “u” sound (as in us) and “a” sound (as in ago)?
Question 2:
What’s the difference between “o” sound (as in got) and “o” sound (as in on)?

Question

What’s the difference between “u” sound (as in us) and “a” sound (as in ago)?
Question 2:
What’s the difference between “o”  sound (as in got) and “o” sound (as in on)?

Answer

#1 "us" [əs] and "ago" [əˈɡoʊ] have the same "uh" vowel (schwa [ə] a soft, short "uh") in General American English (which is spoken in most of the United States and Canada).  I can't address the British pronunciation.

#2 "got" and "on" have same "aw" vowel [ɑ] in General American English.  A narrow phonetic transcription shows that the vowel is nasalized [ɑ̃] in "on" but not nasalized [ɑ] in "got."

on

Answer

There is none... well, to my native ears. 😅

There may be a formal nuance but the International Phonetic Alphabet will help you clarify.

Answer

If there is a difference, most native speakers can't hear it.

If it's any consolation, my American ears have a hard time recognizing the difference between Mandarin 去 (qu) and 吃 (chi).

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