Attacks of Opportunity?

[JimmyNids]

Single melee strike is standard action, spells are standard actions(mostly). My DM insists if you cast a spell you can't make attacks of opportunity, regardless of being a magus holding a sword and casting. Anyone have a point of official reference either way proving one of these conclusively? If I cast a std spell but can't make an AoO even though armed, why should a fighter who uses the same time to shout while swinging a sword??

[DM Barcas]

There is no general rule whatsoever regarding AOOs and the previous turn's actions. You can take an AOO so long as you are not flat-footed, properly provoked in a square you threaten, and have not taken an AOO yet this turn.