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.

Salt is completely water soluble, unlike only trace amount of what is in oregano. That being the case, it will completely saturate anything that is wet, therefore it can completely consume and pair with every bite, whereas oregano remains in chunks and can be distinctly tasted alone.

Salt has a neat ability to bring out and enhance flavours in other foods. If the food tastes "salty", you're probably adding too much. If you use something like Kosher salt, you don't actually need to use very much to add the flavor.

Here's a decent article on it.

Alton Brown talks about it in many different episodes.

This is because salt is a natural flavor enhancer, which basically brings out the flavor of whatever you are putting it in. Different salts also give different flavors, depending of the minerality in them.

Oregano actually does not relevantly TASTE of anything, apart from a negligible bitterness (you would notice it if you tried eating a can of dried oregano straight :) ). It works on the aroma level of things. Also, as with most aromatic ingredients, the aroma can change in character and potency when heat is applied or other chemical reactions during cooking.

Clean salt is not involved in aroma at all, but in the balance of the six basic tastes (sweet, salty, bitter, sour, umami, fat). There is a paradox regarding your impression: Aromas tend to actually change when heat or other cooking methods are applied (by evaporation, solution, chemical changes), while salt stays salt (either as a solid crystal or as Na and Cl ions) throughout any preparation process that still leaves you with an edible result. However, unlike aromas, basic tastes emphasize and/or cancel each other depending on their balance.