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