Why does salt taste different in different dishes?
Two properties of salt that oregano doesn't have:

water solubility: you drop salt in water, you put it on chicken, you put it in tomato sauce.  Most or all of it ends up dissolving, making it easier to mix.

osmosis: related to above.  Sprinkle some salt on raw meat.  Come back in 5 minutes and you'll see beads of liquid on the meat, almost like it was sweating.  That's a combination of salt dissolving into the liquid on the surface of the meat, and osmosis pulling less salty water out of the cells of the meat to balance the salty water on the surface.  In 10 minutes, some of that liquid will have disappeared.  Part of this might be due to evaporation, but part of this is due to the liquid pulling salt into the cells, which changes the equilibrium and allows for the cells to draw liquid back in (principle behind brining meats).

Salt has a natural way of penetrating organic matter that oregano and many other seasonings don't.  There are other mechanisms at play too--salty foods can make you salivate, which may make something feel "juicier" in your mouth.  I'd highly recommend the Good Eats episode on salt as a start.