Pickaxe said:
Some questions about the nature of offhand weapons came up in a discussion I was having with a friend.
1) Let's say a ranger with a regular longsword and a magic shortsword moves and makes an attack. Can he make his single attack with the shortsword? If so, does he suffer any penalty because this would be his offhand weapon?
2) Can the same ranger make an attack of opportunity with the shortsword? If so, does he suffer any penalty?
3) If a character has a shield (possibly with spikes) but no weapon drawn, could they make an attack of opportunity using a shield bash? Since a shield is always considered to be offhand, do they suffer a penalty?
--Axe
Pretty big can of worms, here.

There are no rules for handedness (whether a person is "right-handed" or "left-handed") in D&D. There are rules for using a weapon in the "off-hand", but that term only comes up when discussing TWFing to get an extra attack during a full attack.
"Off-hand", therefore, means "extra attack when using TWF". It definitely does not mean "left hand". For example, a person TWFing with a greatsword and armor spikes is using a greatsword with *both* his hands, and slamming/kneeing/etc. with the spikes as an "off-hand" attack--even though that off-hand attack is not made with a hand-held weapon.
There's a quote in the Damage section of the SRD that references the TWF rules: "Off-Hand Weapon: When you deal damage with a weapon in your off hand, you add only 1/2 your Strength bonus." This quote makes things confusing by mixing "off-hand weapon" with "weapon in your off hand". As far as I know, this is the only reference to an off-hand weapon actually being held in the off-hand. Note that there is no definition in the rules for "off-hand" alone--only references to "off-hand attacks" (in other words, the extra attack made when TWFing).
So, all that said:
1) Yes, he can make a single attack with the shortsword. In this case there is no TWF (he's not making an extra attack during a full attack--he's not TWFing), so the short sword isn't an off-hand attack. He gets full Str bonus to damage, and there is no TWF penalty.
2) Sure. If the ranger had a longsword in one hand, a short sword in another, armor spikes, a spiked gauntlet, and the feat Improved Unarmed Strike, then he'd be threatening with five weapons. If given an AoO, he could choose to attack with any one of those weapons. Regardless of his choice there'd be no attack penalty, and he'd get full Str bonus.
3) Shields don't always have to be off-hand attacks. The official FAQ addresses this exact question:
FAQ said:
Q: Can a character make a shield bash attack using the
shield as a primary weapon or can it be used only as an offhand
weapon?
A: While the rules describe a shield bash as an off-hand
weapon, that’s simply an assumption (that your primary hand is
holding a weapon). There’s nothing stopping you from
declaring your shield bash as your primary weapon. Of course,
that means that any attack you make with your other hand
becomes a secondary weapon.
Again, the term "off-hand" only applies when TWFing. If you're making just a single attack, then whatever attack you're making is just a normal attack--regardless of whether the weapon used to make the attack is hand-held, regardless of whether the weapon is held in the right or left hand.
So if the character makes the AoO with the spiked shield he'd attack at no penalty and get full str bonus to damage.
-z