The description of the Oversized Two-Weapon Fighting feat says (Complete Adventurer, p. 111):
You are adapted at wielding larger than normal weapons in your off hand.
Prerequisite: Str 13, Two-Weapon Fighting
Benefit: When wielding a one-handed weapon in your off hand, you take penalties for fighting with two weapons as if you were wielding a light weapon in your off hand (see page 160 of the Player's Handbook).
Special: A fighter may select Oversized Two-Weapon Fighting as one of his fighter bonus feats.
I have seen some discussions (like this one) and it seems most people are taking this as "just adding 1 point of dmg in average on off hand, not worth it". But when I took a deeper look at it, it seems wielding a one-hand weapon on off hand can bring some other benefits including:
- Removing the -4 panelty vs. Sunder
- Removing the -4 panelty vs. Disarm
- Can apply power attack (-5/+5 on both hands is essentially the same to -5/+10 on one hand, so you are just trading AB -2 with the extra weapon damage on your off hand. Not a bad trade if you are doing TWF already.)
- Having more hp vs. Sunder (low-level only, people are relying on enhancement bonuses at higher level)
- Open up additional off hand weapon options (especially when you are getting items generated by DM rather than creating by yourself)
I wonder why those benefits are overlooked especially when some monsters do have Improved Sunder or Improved Disarm in their feat lists (Water and Earth Elementals, giants, etc.; some even have those written in their default tactics, e.g. Titan), and some monsters actually have Sunder as an effective weakness (e.g. Kraken, Hydra, etc.). I feel that not taking this feat would likely let those TWF meleers say goodbye to their off hand weapons at the first round. (And I am feeling light weapons are really not very safe to play for frontline PCs.)
(It can potentially gain more dmg stack with Monkey Grip (Complete Warrior) but let's not consider it since it requires one more additional source.)