The agents released from oak wood are actually quite good - the tannins aren't too bad if it's a look you're after, otherwise a fine mech filter floss and some carbon will also help reduce yellowing for the first month or so that you have the piece in the tank.... I would follow Joels advice above in regards to preparing it - time is your friend, especially with larger pieces.
It's not just tannins - the agents released are shown to display useful anti-bacterial properties and I have seen this myself in fish with split fins etc, the rate at which they heal is quite significantly increased whilst the probability of secondary infection is considerably reduced....
You can see an aquascape I had recently here- http://www.cichlid-forum.com/phpBB/view ... highlight=
I used freshly collected Oak leaves from my local woods, boiled to help them sink but also to ensure that any undesirables don't enter the tank. I've never had any problems, I've also collected my own driftwood and again, never had problems, as long as you have the patience to prepare the piece properly then it's win win.
In case you don't read through it:
1. Only collect hardwood (and of the hardwood's I would avoid Poplar and other soft hardwood's like it.)
A good list can be found here: http://www.ncsec.org/cadre2/team18_2/st ... rdSoft.htm
2. If it's rotting it's no use.
3. If it releases milky sap do not collect it.
4. I look for dry pieces that can be cleaned with a good scrub.
5. I avoid pieces that have splits in them (all sorts of #%$& gets in there).
6. Sand blasting is really effective... Hint hint. It helps bring out the softer wood on the surface leaving the harder wood in ridges etc - much more interesting contours.
7. Softer woods can be treated (much like mopani is) by burying them in the ground and letting them bake (under hot sun in their case) for a good couple of weeks, this makes them a lot more resistant to rot and also makes them that bit more durable.
8. Soak it, for a long time, you want to get as much of the tannins out as possible and as much water in (so it sinks).
9. If the soaking is taking you too long then you can boil it up a few times each day or every few days to really speed up the rate at which the tannins are released as well as the saturation of the wood - it is still going to take a fair while, especially with dry or larger pieces.
I generally say avoid softer woods because it is with softer woods you'll have the biggest mold/break-down issues in the tank, it doesn't take very long either.