I've played a few other CCG/LCG games. In Warhammer:Invasion, you can play cards in response and basically the actions resolve LIFO (Last In, First OUT).

From reading the rules, it looks like that is not the case.

For example, if my opponent played Force Choke (http://www.cardgamedb.com/index.php/starwars/star-wars-card-spoilers/_/core/force-choke-core-23-6), I could not save my 1 HP unit by playing OMDH (http://www.cardgamedb.com/index.php/starwars/star-wars-card-spoilers/_/core/our-most-desperate-hour-core-3-5) in response, since my action would occur AFTER his action?

I'm guessing Interrupts and Reactions are for this purpose?

That's my understanding as well. If it were an interrupt, you'd be good to go.

Interupts are the only thing that can slide in before a card is triggered. All else is a one-card-at-a-time back and forth.