Its called a worm hole. Cant always see them while installing. Pump epoxy into it (after cleaning) to give some structural strength and then if theres still a little lip you can fill with colored wood putty.

Thats the fastest/cheapest/easiest. Replacing the board is the alt. way.

I guess I was hoping for more of a "how-to". Is there a particular epoxy to use in order to fill it? Should i tape off the edges of the floor? How do i get the epoxy inside - press it in?

I can see down to the subfloor, so do i just fill it until its just below the wood floor and then put putty over? I don't really want to sand the epoxy because that would probably take a lot of effort.

Ok. Tape around the hole.
Most any clear epoxy will work. A trick you could use since you can see to the subfloor is find a old rag. Cut just a little piece and shove that in the hole first. This gives the epoxy something to butt into and not run all over the subfloor.

3m makes a good epoxy and they even have a straw that it attaches to the cartridge which will give you a precise control when filling.

Fill the hole til you can see the epoxy come close to the top. Then stop. Let it set up and cure.

You should mow only have like 1/16th to 1/8th gap btween the top of the hole to the top of the epoxy.

Now you find wood putty(waterbased) and find a color that matched best. And then fill the remaining gap til its flush with the floor. Wipe the edges with a damp rag and bingo. Hole fixed.

Real good answer!

thanks again for helping me out this is great! So I couldnt find a 3M, so I used a loctite brand. It claimed 1 minute cure but its been 5 hrs and its not rock hard. I think i should be ok for the most part though.

I bought the wood putty and noticed that it doesnt dry hard, should i have gotten the wood filler instead? I have a big dog and Im concerned if it doesnt dry he will rip out the putty. I bought the minwax brand. Your additional help is much appreciated!!

It wont dry rock hard but it should stiffen up enough. This is one area you can play around with

Id say stay away from the oil based just because its messy and usually requires finish applied to it.

Woodwise is a good brand that many people use for wood filler. Many colors and is prefinished.

But unless you have already opened the one you bought and cant return it id just use what you have. The worm hole is small enough where if you need to add more putty in a few months its not a big hassle.

Glad it worked out for you!

I would have pinned a straight edge to the floor and run a 1/4" router bit 1/4" deep, removing the brown spot. I would have fitted a piece of matching oak into the slot and stuck it with wood glue, left it proud, then sanded it flush. With a little touch up finish, it would have nearly disappeared.

Thats def another way to fix it but requires way too much in terms of tools, skill, stain match, finishing time, knowledge, materials.

I would scrape the dark margin of the wormhole down to new wood, because clear epoxy will take on the color beneath, so your filler will look dark, too, like the void.
Casey