I don't have the botannical knowledge to comment on the many woods listed previously, but I certainly would not use coniferous (evergreen) wood like pine, cedar, hemlock, spruce (I know, this last isn't there, just another one that occurs to me). These have resins and saps that can kill fish. Cedar also is a soft wood that therefore decomposes/rots quickly under water; it is fine in a natural creek, but not in an enclosed aquarium.
Using wood from the wild is always a risk; you can't know what toxins it may have come into contact with, like pesticides, fertilizers, oils, chemicals... and these can leech out over time and kill everything in the tank.
Suitable wood (oak is one, it is a hardwood) is of itself safe (remember the above-mentioned unknowns with any wood) but needs to be completely dried, then soaked or some bake it. This kills pathogens, parasites, etc., or should. Then you need to waterlog it, because it will not sink dry.
Wood purchased from a reputable and reliable fish store is certainly less risky, if expensive.
Edit: Just spotted an earlier thread on this topic, related anyway, you might want to follow it: http://www.tropicalfishkeeping.com/f...pare-dw-39586/