Howdy,
Oak leaves are an old-school favorite! I have never used them myself, though. From what I've heard, oak leaves are pretty resistant to rotting (that's why they are used as compared to other leaves). They are slightly acidic and could reduce your pH value at first. You may monitor that, and please post updates. The leaves will not last for years, though, and as stated above, you should keep an eye on your water parameters. On the other hand, it shouldn't be too much work to siphon them out in the Fall and add new leaves...
Please post some pics and reports on your observations and water chemistry as the year goes on.
What fish do you keep?
HarleyK