Installing luaun (aka luan) would be a bad move. But before you do anything you have to be sure the subfloor is indeed particle board as you thought. I will bet it is particle board since it was widely used in the '70's and into the '80's, and OSB wasn't.
Tile 4 You LLC Troy, MI
KERDI Shower Specialist ... DITRA Installs ... Product-Method suitability consultation. I have NEVER made a mistake, I thought I did once...........but I was wrong! - PRODUCTS ADVERTISED WITHIN MY POST ARE NOT ENDORSED BY ME!