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 😉

In theory you can do it, but the quality of such an app will be so low, that nobody will use it. And it will certainly not be allowed into the store.

Success in VR is a fairly expensive thing.