must I buy oculus device when I want to develop an oculus vr app

Hi,

You can make a VR app even if you don't have the helmet. Here is a complete tutorial for do it : How to Make a VR Game WITHOUT a VR Headset - YouTube

But if your app become bigger, you will need one for really "feel" your application as the user will feel it.

Please mark this answer as "solution" if that helped you 😉