I sort of had a situation like that, too. Before I was diagnosed, I slept probably 10-16 hours a night if I didn't set my alarm. It was really strange for me, because as a teen I was fine with 7-8 hours (although I have always slept more than the average person, I think - I love my sleep

). I ended up having throid disease and a b-12 deficiency along with my Celiac disease, all of which cause fatigue.
After treating all of that, it started to get better, but I was still having problems waking up in the morning. I was exercising and taking my vitamins, etc. I asked my doctor and she said that vitamin deficiencies can cause depression, but also major life changes (like totally revamping your diet and isolating yourself from social activities that include food) can trigger depression. For some people, depression can cause insomnia, but for others it can cause them to sleep too much. I decided to see a therapist, and shortly I went back to being fine on 8 hours sleep. I have been able to wake up in the morning just fine. It's been great. Even now that I am pregnant, I am tired, but not as tired as I was back then.
So, anyway, if you have other symptoms of depression, it might be something worth looking into. However, if oversleeping is the only symptom (or if the symptoms are only physical), it is probably not depression. Don't let doctors tell you it is depression unless you really feel like you are depressed.
Back before I was diagnosed, my doctors didn't take me seriously and diagnosed me with depression without looking at anything else, solely based on the fact that I was oversleeping, had GI symptoms, and was losing weight. I was like, "funny, I don't feel depressed". I refused treatment and went to another doctor. It was only after then (two years after then, but nonetheless) that I was diagnosed with celiac disease, Thyroid Disease and the vitamin deficiencies. I'm so glad now that I did that. It makes me angry when I think of how much longer I would have suffered before I got a correct diagnosis if I hadn't done that.
Anyway, sorry this is so long.
-Sarah