There are many different oils, which basically have a soothing and softening effect. Of course it is meant for the tips, but it makes thin hair easily greasy... I still use some oils nevertheless. But I would recommend you urgently to think about your nutrition and take hair supplements. I did this and it really helped.
Sounds like u should see a doctor and get blood tests. Then you can see if u are not getting enough of any nutrienets and your doctor can tell u if u should be taking supplements and what dose you should take.
Or if perhaps you have an underlying medical issue (like a thyroid problem) which needs addressed. Either way, you should see a doctor and get a check-up.
Also sometimes it can simply be you are handling your hair too roughly. Treat it like its precious gold! If its from brushing/combing let the hair dry since wet hair is more delicate and then use a detangler in your hair dried hair, you can add that when its a little damp but not soaking.
I solve the problem of getting tangles out by putting a lot of conditioner in my hair in the shower and combing through with a wide tooth comb. Then I rinse. I don't comb it again. I don't brush it. It stays tangle free most of the time. On the rare occasions when I DO get tangles I comb through with my fingers! For me, combs/brushes = FRIZZ.