Oklahoma State isn't playing LSU for the national championship because Oklahoma State isn't in the SEC. We can parse any number of lesser facts -- computer rankings and human polls, strength of schedules and margins of victory -- but the only fact you need is that one:
Oklahoma State isn't in the SEC.
And so Oklahoma State got screwed.
The Cowboys got screwed by a system that I'm not willing to call deceitful, because "deceitful" would imply intent -- and I don't think there was any conscious intent to screw Oklahoma State. No, Oklahoma State got screwed inadvertently by people and companies that don't have any idea how unfair they're being.
Oklahoma State got screwed by voters, who are influenced by the two biggest television networks in college football. I work for one of those networks, CBS, and I'm telling you, I don't believe for a second that my network intentionally, consciously conspired to keep Oklahoma State out of the national championship game, just as I don't believe ESPN consciously conspired to keep Oklahoma State out of the national championship game.
That's just the way it worked out.