#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." Pronunciation from the New Oxford American Dictionary us | BrE ʌs, AmE əs | ago | BrE əˈɡəʊ, AmE əˈɡoʊ | got | BrE ɡɒt, AmE ɡɑt | on | BrE ɒn, AmE ɑn, ɔn | on

There is none... well, to my native ears. 😅 There may be a formal nuance but the International Phonetic Alphabet will help you clarify.

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).