A solid healthy piece of oak would never collapse like that. You need to figure out what is going on before you fix it. Look at the underside if you can, break away any of the edges of the hole that come out easily (preferably without getting into the surrounding pieces) and try to figure out what you're dealing with. Look hard for moisture or evidence of insects. If you find anything suspicious, those problems must be dealt with first.
If ,however, the hole is pretty localized and the rest of the floor feels dry and solid, you can reasonably assume that it was an isolated defect in that piece and proceed with a patch. Here's a pretty decent tutorial on patching hardwood flooring: