From an optimization point of view, Oversized TWF is a trap. Take it only for the flavor. Why? Because a bastard sword deals an average 2 points of damage more than a shortsword, and 2 points of damage don't justify a feat, IMHO. Place this feat on the same level than Weapon Specialization, with the slight difference than OTWF doesn't open the way to Weapon Mastery and Weapon Supremacy (which you don't care about either 'cause you're no fighter).
Regarding damage, the broadly accepted theory is that TWF is only fine if you have an alternative source of damage, such as Sneak Attack, Favored Enemy, or the Duskblade's Arcane Channeling. I'm not quite sure if this is always true, but let's compare Rod the Ranger, and Bob the Lion Totem Barbarian, two lvl 16 twin brothers with 26 STR, and an orc guarding a chest in a 10'x10' room.
Rod has TWF, ITWF, GTWF, as well as +8 on damage vs. orcs from Favored Enemy. He's using a pair of +3 collision shortswords, 'cause you're pretty rich when you hit lvl 16.
16 BAB, +3 Enh, +8 Str, -2 TWF, means +25 to hit, for a total of 25/20/15/10 (1d6+24 - 8 Str, 8 Fav, 3 Enh, 5 Col) with his primary hand and 25/20/15 (1d6+20) with his off-hand. Assuming he always hits (yeah, right), that's a damage range of 163-198, average 180.5 (without Favored Enemy, remove 56 from each total).
Bob has Power Attack, Improved Bull Rush, Shock Trooper, as well as his Pounce ability for being a badass Lion Totem barbarian, one of the most broken class variant evah', and he's wielding a +5 collision greatsword, costing roughly the same as his brother's short swords.
16 BAB, 8 Str, 5 Enh, -5 (for example) Power Attack, means +24 to hit, i.e. 24/19/14/9 (2d6+32 - 5 Enh, 5 Col, 12 Str, 10 PA), damage range 136-176, average 156. Not as good as his bro, but not enemy-dependant. Now let him Pounce...
+2 to hit from a charge, and all his To Hit penalty from PA goes to AC (Heedless Charge maneuver from Shock Trooper), allowing him to use PA at -16, 'cause he's angry. He's not ragin yet.
16 BAB, 8 STR, 5 Enh, 2 Charge, means +31 to hit, i.e. 31/26/21/16 (2d6 + *drum roll* 54), damage range 224-264, average 244. Ouch. And that's without using the quite widely spread Leap Attack, or without raging. And without the bonus from Favored Enemy.
Using a Move/Partial Charge Leap Attack routine each round, Bob could hit at +31, dealing 2d6+86 damage on a single strike.
I'd say it's quite equivalent, even slightly better for PA, even though both are quite situation-dependant. Now consider that:
- TWF suffers more from Damage Reduction than standard full attacks
- TWF is MAD (yeah, you need a high Dex as a prerequisite, not your ranger's usually highest attribute if you go the TWF way)
- TWF has much less feat options
On the whole, it's not bad, but not really optimized. TWF only really shines with +16 BAB and an alternate source of damage, which you do not get if you stick to a single class, with the exception of Duskblade. Then again, there are builds like Ftr 4/Rog 16, or Swift Hunter (Ranger 4/Scout 16). But then again, we're talking high-level comparisn; at low levels, the problem is quite different and the TWF penalties even more painful (think of it, a good Fighter may get Leap Attack and Shock Trooper upon reaching level 6...). If you go Gestalt, the options are somewhat better and it might even beat THW (think Ftr//Rog gestalt with Adaptable Flanker, Keen kukris and Telling Blow, for example, or Ranger//Duskblade). But on the whole, I'd say THW has the upper hand regarding damage.