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