How does threatening reach work.
If you shift you are okay, but you move within X squares of reach you get an OA on you.
How about when you "First" move in to attack, do you get an OA on you then ?
If you shift in that distance then you are fine.
But if this does trigger an OA, what if you charge in instead ?
The intention of the move is irrelevant to OAs.
...pet peeve. 'OA' as an acronym makes it harder to talk about opportunity actions. Just sayin'... we need a new acronym.
What matters is this:
Did the person leave a square that is eligible for an opportunity attack?
Is the person doing something that makes them immune to either opportunity attacks or opportunity actions?
If the answer to the first is yes, and the second is no, then an opportunity attack may occur if the reactor hasn't taken one yet this turn.
So... Threatening Reach extends the 'adjacent' rule for opportunity attacks to within reach. That's pretty simple.
So:
"How about when you "First"
move in to attack, do you get an OA on you then ?"
It depends on how you move. If you use a power that allows a shift or a teleport, no opportunity attack can occur. If you use a walk action or a run action or a crawl action, then certainly an opportunity attack may occur.
"But if this does trigger an OA, what if you charge in instead ?"
Ask the question 'Does this render me immune to opportunity attacks or actions?' Generally, charges do not render that immunity, however there -are- circumstances where a piece of equipment or a specific power might.