Here is the full list of all 13 Oxenfree achievements. It takes around 12-15 hours to unlock all of the achievements on Windows.
Slap Ren in the face.
Play the whole game without saying anything.
Complete the story.
Bring Michael back from the dead.
Jonas is your new step brother.
Collect 4 of Maggie's letters.
Collect 8 of Maggie's letters.
Collect all of Maggie's letters.
Find all hidden frequencies.
Make enemies with all of your friends.
Jump the crazy chasm in the woods.
Get Ren and Nona to date.
Keep Ren and Nona from dating.