My husband and I bought a 1942 house recently. We're considering replacing the linoleum flooring in the bathroom. When we take the transition strip off we see the linoleum sitting on a thick sheet of particle board. Underneath the particle board is a fir floor. We thought the fir floor was the sub-floor, but now we're not sure for a couple of reasons. The first is that the fir is smooth and level with the oak flooring in the rest of the house. Second, when we look up from the basement, the sub-floor runs diagonally. Is this fir actually a floor that could be refinished? Not sure why it would be level with the oak, if that's not the case. We have the same thing going on in the kitchen, as well. Thanks for any insight!

This was common in that era house. In the areas the builder judged most likely to be covered with linoleum they used a less expensive tongue & groove flooring. My 1922 vintage house is done in similar fashion.
Bill