The first one is pronounced with an O shape with the mouth (like you would do with the word oh), and the others with more of a smile shape (like with the word see). It’s impossible to pronounce one like the other.
I’m not a native English speaker and I gave up trying to pronounce th (father, through). Although I can hear the difference.
> I’m not a native English speaker and I gave up trying to pronounce th (father, through). Although I can hear the difference.
Why can't the Québécois count to four? Because there is a tree in the way.
This has to be a regionalism because there're strictly identical to me, eg. in "Un train." /œ̃tʁɛ̃/ I say the two vowels exactly the same way.
After a cursory search it seems my Parisian-ish accent is at fault: https://fr.wiktionary.org/wiki/Annexe:Prononciation/fran%C3%...