A confusing plot backed by endless boob jokes and fanservice. Oh. And a unique battle system.
This means that aside from a few nods to Original Generation, this game has almost nothing to do with any other Super Robot Taisen game.
There is a plot here, but you'll soon notice that the focus here isn't the plot at all, but the battle system. At first, it may seem overly simple. You press A, and your character bursts into action, perform 20-30 hit combos in a single attack. The point here, however, is much like a fighting game. Keep your opponent in the air and perform as many combos as you can. However, instead of entering specific inputs, all you have to worry about is the timing of your attacks. Let your enemy hit the ground, and you'll find yourself eating a combo yourself.
Much like many action RPGs, OG Saga has something akin to an action meter known as the COM meter. Each attack takes up a certain COM %, allowing a total of up to 100. Here's the catch: the order of your attacks is determined by the player in between battles. There's also a practice mode for all 3 enemy weight classes, allowing the player to experiment with attacks to figure out the best combos. It is deeply satisfying to pull off a 200 hit combo on a boss.
The other emphasis is on the jokes, which I will not spoil for you here. Know that this game is essentially fanservice to any SRT fan. There will be much juggling of objects besides your enemies, especially in special attacks... if you get what I mean.
It's not without faults though. Boss battles are more or less battles of attrition. You will NOT win without items, and lots of them at that. You might have a couple thousand hp, but bosses will have a couple hundred thousand. You'll be constantly using items and spells to overpower bosses. Sure, it's cool and fun the first couple of times, but as the battles drag on, you'll be doing the same combos, casting the same spells, and using the same items over and over until you've finally depleted the boss's hp.
It's a decent enough game, moved along by its wit and the flashiness of its combat. If you bother digging, you'll find a fair bit of substance beneath all the eyecandy.