I have two different blades (which are almost new) for my mitre saw. One has 24 teeth, the other has 48 teeth.
Which blade should I be using to cut oak floor boards?
I'd use the 24 tooth blade, but be sure it is a carbide type and sharp. Since all your cuts are end cuts and will be covered with baseboard trim, so getting an ultra smooth cut is not that important. Obviously, you don't want to see any large chips on the cut edge, so do your cuts slowly and smoothly. Save your 48 blade for visible finish cuts on softer wood. If you do plan to do any precise finish work, moldings etc, consider investing in an 80 tooth blade for that type of work. Good Luck.
Sorry I am going to disagree somewhat with "shirlock homes" answer.
Given the choice between a 24T & 48T saw blade to perform the task (cross-cutting oak floorboards), I would most definitely go with the 48T blade, you will get a much cleaner cut (& less tear-out on the underside).
I would use the 24T blade on softwood (especially framing timber), mainly where you will be hiding the softwood behind something else.
Totally agree with "shirlock homes" on getting yourself an 80T (or even an 100T) blade for really precise finish work and making sure you use (sharp) carbide type tooth blades.
Below is a handy reference for helping someone select the right saw blade (well worth a read IMHO):
If you read the above referenced document, you will see a 24T or 48T blade could be used for cross-cutting hardwood floorboards. I would just rather go with the 48T given the choice between the two.