There are professional waterborne wood floor fillers in "knot brown" color that would work here. Or, as I have done on occasion...3M makes a clear epoxy called DM100 that is used in hardwood flooring repair. They make an inexpensive mixing/delivery tool that makes the product easy to mix and apply exactly where you want it. You fill the void an then fill again, if necessary, to bring the filled surface level or just a little proud. You can stuff the bottom of the filled space with sawdust to reduce the amount of epoxy you'd have to use. With a waterborne wood floor filler that would not be necessary...it's cheap enough.

Either method will probably give you good results, but like in anything and wood flooring "test" before you do anything to the floor.

Filler to look for is Timbermate or Woodwise brands.

Thanks. Appreciate the help.

You're welcome.

Very nice flooring you have there. White oak makes for a strong and attractive floor.