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.
Complete the story.
Jonas is your new step brother.
Keep Ren and Nona from dating.
Jump the crazy chasm in the woods.
Play the whole game without saying anything.
Bring Michael back from the dead.
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.
Get Ren and Nona to date.