Shield bashing and AOO

[VinnythePooh]

My gripe with my DM: I'm carrying a spiked shield as a cleric and have a wand in my other hand. Can I use the shield as a weapon, since it is often my only weapon, as as a primary attack and thus be able to make attacks of opportunity with it?

[MaxiDuRaritry]

In the SRD under both Weapons and Armor:

Shield, Heavy or Light: You can bash with a shield instead of using it for defense. See Armor for details.

Spiked Shield, Heavy or Light: You can bash with a spiked shield instead of using it for defense. See Armor for details.

Masterwork Weapons: [snip] Even though some types of armor and shields can be used as weapons, you can’t create a masterwork version of such an item that confers an enhancement bonus on attack rolls. Instead, masterwork armor and shields have lessened armor check penalties.

Shield, Heavy, Wooden or Steel: You strap a shield to your forearm and grip it with your hand. A heavy shield is so heavy that you can’t use your shield hand for anything else.

Wooden or Steel: Wooden and steel shields offer the same basic protection, though they respond differently to special attacks.

Shield Bash Attacks: You can bash an opponent with a heavy shield, using it as an off-hand weapon. See Table: Weapons for the damage dealt by a shield bash. Used this way, a heavy shield is a martial bludgeoning weapon. For the purpose of penalties on attack rolls, treat a heavy shield as a one-handed weapon. If you use your shield as a weapon, you lose its AC bonus until your next action (usually until the next round). An enhancement bonus on a shield does not improve the effectiveness of a shield bash made with it, but the shield can be made into a magic weapon in its own right.

Shield, Light, Wooden or Steel: You strap a shield to your forearm and grip it with your hand. A light shield’s weight lets you carry other items in that hand, although you cannot use weapons with it.

Wooden or Steel: Wooden and steel shields offer the same basic protection, though they respond differently to special attacks.

Shield Bash Attacks: You can bash an opponent with a light shield, using it as an off-hand weapon. See Table: Weapons for the damage dealt by a shield bash. Used this way, a light shield is a martial bludgeoning weapon. For the purpose of penalties on attack rolls, treat a light shield as a light weapon. If you use your shield as a weapon, you lose its AC bonus until your next action (usually until the next round). An enhancement bonus on a shield does not improve the effectiveness of a shield bash made with it, but the shield can be made into a magic weapon in its own right.

Shield Spikes: When added to your shield, these spikes turn it into a martial piercing weapon that increases the damage dealt by a shield bash as if the shield were designed for a creature one size category larger than you. You can’t put spikes on a buckler or a tower shield. Otherwise, attacking with a spiked shield is like making a shield bash attack. An enhancement bonus on a spiked shield does not improve the effectiveness of a shield bash made with it, but a spiked shield can be made into a magic weapon in its own right.

Nothing there says anything about not using shield bashes for AoOs, and it should, if using a shield bash (with or without spikes) is disallowed. So even if something says bashing can't be used on AoOs elsewhere, that's not the primary source for shield bashing, and thus shouldn't count.

[VinnythePooh]

But it says everywhere that one can use shield bash as an offhand attack and that's why my dm disallowed the AoO. He sucks

[MaxiDuRaritry]

But it says everywhere that one can use shield bash as an offhand attack and that's why my dm disallowed the AoO. He sucks

Here's the full text for AoOs:

Attacks Of Opportunity
Sometimes a combatant in a melee lets her guard down. In this case, combatants near her can take advantage of her lapse in defense to attack her for free. These free attacks are called attacks of opportunity.

Threatened Squares
You threaten all squares into which you can make a melee attack, even when it is not your action. Generally, that means everything in all squares adjacent to your space (including diagonally). An enemy that takes certain actions while in a threatened square provokes an attack of opportunity from you. If you’re unarmed, you don’t normally threaten any squares and thus can’t make attacks of opportunity.

Reach Weapons
Most creatures of Medium or smaller size have a reach of only 5 feet. This means that they can make melee attacks only against creatures up to 5 feet (1 square) away. However, Small and Medium creatures wielding reach weapons threaten more squares than a typical creature. In addition, most creatures larger than Medium have a natural reach of 10 feet or more.

Note: Small and Medium creatures wielding reach weapons threaten all squares 10 feet (2 squares) away, even diagonally. (This is an exception to the rule that 2 squares of diagonal distance is measured as 15 feet.)

Provoking an Attack of Opportunity
Two kinds of actions can provoke attacks of opportunity: moving out of a threatened square and performing an action within a threatened square.

Moving
Moving out of a threatened square usually provokes an attack of opportunity from the threatening opponent. There are two common methods of avoiding such an attack—the 5-foot step and the withdraw action.

Performing a Distracting Act
Some actions, when performed in a threatened square, provoke attacks of opportunity as you divert your attention from the battle. Actions in Combat notes many of the actions that provoke attacks of opportunity.

Remember that even actions that normally provoke attacks of opportunity may have exceptions to this rule.

Making an Attack of Opportunity
An attack of opportunity is a single melee attack, and you can only make one per round. You don’t have to make an attack of opportunity if you don’t want to.

An experienced character gets additional regular melee attacks (by using the full attack action), but at a lower attack bonus. You make your attack of opportunity, however, at your normal attack bonus—even if you’ve already attacked in the round.

An attack of opportunity "interrupts" the normal flow of actions in the round. If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next character’s turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a character’s turn).

Combat Reflexes and Additional Attacks of Opportunity
If you have the Combat Reflexes feat you can add your Dexterity modifier to the number of attacks of opportunity you can make in a round. This feat does not let you make more than one attack for a given opportunity, but if the same opponent provokes two attacks of opportunity from you, you could make two separate attacks of opportunity (since each one represents a different opportunity). Moving out of more than one square threatened by the same opponent in the same round doesn’t count as more than one opportunity for that opponent. All these attacks are at your full normal attack bonus.

Why would making AoOs with your off-hand make a difference? Nothing prevents off-hand AoOs.

[Darg]

But it says everywhere that one can use shield bash as an offhand attack and that's why my dm disallowed the AoO. He sucks

It's a 3.0-3.5 update mistake. Off-hand weapons no longer exist. It's a legacy term taken out of context to make understanding the two weapon fighting rules easier. In 3.5 off-hand weapons don't exist outside of those rules.

In 3.0 when creating a character you would designate a main hand and off-hand and it would be permanent unless you got the ambidexterity feat. This off-hand would always have a -4 penalty and only ever receive ½ your STR modifier. 3.5 doesn't do this.

So when the shield bash rules state you make an off-hand attack, it's referencing rules that don't exist anymore (even though the term and rule is defined in the glossary, it's never applied anywhere). Thus, you can make shield bash attacks with any hand in 3.5 and it doesn't have to only be during TWF full attack.

Edit: also note how TWF never tells you to designate a primary hand or an off-hand. If we want to base the game on RAW, TWF doesn't actually work and can only be used with shield bash attacks because those are the only weapons that would be off-hand attacks.