When people say words like "profesoro", "konas", or "sinjoro", the letter O is very pronounced. It sounds like the English word "oh". When ON appears at the end of the word, people lighten the O so that it sounds like the English word "on".
Compare the words "konas" and "Saluton". People seem to be pronouncing the ON sound differently.
If the letter O sounds like the English word "oh" then shouldn't the letters ON sound like the English word "own"?
I am not sure what people you are talking about, because this is the kind of detail that varies a lot depending on the native language of the speaker (e.g. a French speaker may pronounce o more open if followed by a consonant at the end of syllable).
In theory, o should always have the same pronunciation, whatever its position in the word is. It should be a mid back rounded vowel, but some leeway is accepted as long as the listeners don't mistake it for another sound.
However, oh and own are bad examples because they actually contain diphthongs (combinations of two vowels) and sound more like oŭ.
I think if someone is pronouncing konas to sound like the English words “cone ass” then they are basically making a mistake. It’s probably quite common for English native speakers to do this at the end of words (I do it too sadly), but it’s best avoided if possible. All of the vowels should ideally sound the same in any position. If it seems to you like “everyone” is doing this then you might want to listen to some accents from people with different native languages. In Forvo someone has added a pronunciation of konas and it sounds much more like “con ass” to me. Similarly for profesoro.
Before directly answering your question, I must say that pronunciation in Esperanto is somewhat fluid, which it must be to some degree to facilitate speakers of diverse backgrounds. That being said, I will use phonetic symbols in my explanation as indirect representations often create confusion. Using IPA, the o in "go" is represented by [oʊ̆], as it is composed of [o] and a short [ʊ] (like in "book"). Although this is not detrimental to understanding, this pronunciation in Esperanto is best avoided. For the clearest pronunciation, it is best to isolate the [o] so that it is a pure o sound.
PMEG states:
La Esperantaj vokaloj ne estas diftongoj. Tio signifas, ke ĉiu Esperanta vokalo estas elparolata “senmove”. Oni ne aŭdeble movas la langon de unu pozicio en la buŝo al alia pozicio dum la elparolo de unu vokalo. Ekz. ... O ne kiel “oŭ”.
In English:
Esperanto vowels are not diphthongs. That means that every Esperanto vowel is pronounced "unmoved". The tongue is not audibly moved from one position in the mouth to another position during the pronounciation of a vowel. Ex. ... O not like "oŭ”.
The "on" sound you are talking about, I will assume is [ɔ]. This vowel, being close to [o] and the pronunciation of the letter o in many languages, is also an acceptable pronunciation of o. However, it is best to keep careful that this vowel does not become to close to an "ah" sound, which many English dialects do, and would cause confusion between a and o.
As to why you hear a switch in between the two vowels, in many languages, English included, the quality of a vowel depends on which kind of syllable of situation it is encountered in. In German, for example, o is pronounced as [ɔ] in syllables that end in a consonant, ex. the o in "voll" is [ɔ], but [o] in syllables that don't end in a consonant, ex. the o in "oder" is [o]. This dynamic was at a time advocated in Esperanto by some. It may be acted upon subconsciously by speakers of languages in which this dynamic exists (like English, German, French, etc.), but probably not by those who speak a language with no such distinction (like Spanish, Japanese, etc.). If anyone is pronouncing the o's in words like "konas" and "saluton" differently, it is because they perceive the syllable boundary as "ko-nas" and "sa-lu-ton", but this distinction is not universal and far from necessary for clear speech.
In simple: be mindful that other's pronunciation may differ slightly than yours, but the best pronunciation is a clear "oh" sound, like this [ɔ], or this [o].
P.S.: I did not reference any examples in English as, not only is there no exact correlation to the clear o sound, but the pronunciation of vowels in English vary too wildly to be referenced in this way.
It is virtually impossible to describe the sounds of one language using the sounds of another. This is true for Esperanto as well. This is one of the main reasons that I started my YouTube channel and created the video course Lernu Kun Logano where I focused more on modeling correct pronunciation than explaining it.
Another problem with trying to answer your question here is that even if we both are native speakers of English, we have no way to know for sure that we pronounce the words "cone" or "con air" the same.
In a recent blog post, I describe the O sound as "O as in hope" - but even that is an approximation, so I include links to model pronunciation.
Bottom line, if people are pronouncing the O in konas different from the O in Saluton, they are pronouncing them wrong. In Esperanto, the principle is "one letter - one sound."
The long English O sound in Esperanto is often a dead give away of the persons accent because it's too long and it's not a pure vowel. It's more like a glide because it changes from the start to the end. If you say "kato" like ka-tow then it's probably not right. Pronouncing it like a Spanish O would be tons closer than the English O. So far the only people I've ever heard pronounce the O's this way is Esperanto is English speakers.
