The Oestron was the human tongue even though it had some influence and it was sweetened by the elves. In the beginning it was called Atani and it was the tongue of those men who fought side along side with elves against the Dark Power from the North.
When the Dark Power fell, most of Beleriand was destroyed, and it was given to the men who were the friends of the elves, the ones who fought alongside them and eventually would be permitted to go west beyond the sea to reunite with elves, but not to the imperished lands but to a separate mortal island next to it where men could live, this island was called Numenor, which is where the friends of the elves flourished and became great seamen and powerful beings and had a life three times longer than the normal men, these men were the Numenorean, whom the elves ended up calling Dunedain.
Only the Dunedain knew and spoke the elvish tongue, but their common tongue remained Adunaic (Atani). These men had multiple harbors in the west coast of middle earth, and their sons and daughters stopped speaking the elvish tongue and began mixing the elvish with the Adunaic, which in time got mixed with the men's common tongue, giving birth to the modern men's common tongue that is used in the third age.
After Numenor fell, Elendil (the one who gave birth to Isildur, the one who couldn't drop the ring of power into the flames of mordor, the one who continued the line til Aragorn of the third age became relevant) lead the rest of the Numenorean men back to middle earth, to the western coasts.
I will continue this in the next post, I hope you are interested in the story about men and their language
Map of Numenor