You can take actions in the middle of another action, but any action other than a free action ruins the remaining portion of your initial action.
Relevant rules part here is:
... You can perform one or more free actions while taking another action normally...
Now if it has to be pointed for free actions it means you can't generally take another action normally while you perform another action.
Some examples: you end your movement when you attack and remaining portion is wasted, if you do something while you cast a spell it fizzles, you can't continue full-attack after you move your speed in the middle of it even if you have additional move action on your turn. Some of those examples can be changed with exceptions, but an existence of exceptions only strengthen general prohibition.
So for your example Largo can take an attack of opportunity, but then he can't continue his charge. Or he can avoid using attack of opportunity and either continue his charge if path isn't now blocked or attack Count as if he charged him if Count is in the way.
Also, some people may say attacks of opportunity aren't actions. Sure they lack action type. But consider dazed creature, the only effect of the condition being: "a dazed creature can take no actions". Would you allow it to make one?
Attacks of opportunity are also listed as an exception here:
The Combat Round
... When a character’s turn comes up in the initiative sequence, that character performs his entire round’s worth of actions. (For exceptions, see Attacks of Opportunity and Special Initiative Actions.)
meaning they are also actions, but aren't performed on a character’s turn.