Moving Provokes: If an enemy leaves a square adjacent to you, you can make an opportunity attack against that enemy. However, you can’t make one if the enemy shifts or teleports or is forced to move away by a pull, a push, or a slide.

Trigger: Opportunity actions allow you to take an action in response to an enemy letting its guard down. The one type of opportunity action that every combatant can take is an opportunity attack. Opportunity attacks are triggered by an enemy leaving a square adjacent to you or by an adjacent enemy making a ranged attack or an area attack.

Thanks, to clarify my question: if the second character stays in adjacent squares - moving from one to another as he gets to the back of the monster to flank - would he be subject to an opportunity attack? Or, to put it another way, would moving from one adjacent square to get to another one be considered "leaving?"

Thanks for the clarification. I'll explain this to my players next week. I'm sure the rogue will be bummed to learn that he can't waltz past the ogre for a backstab without getting bashed first.

As a side-note: the other thing I realized re-reading the movement rules: a character can move right thru a monster's space, but will provoke an opportunity attack in doing so (unless the monster is two sizes larger or two sizes smaller than the character). I thought they couldn't pass thru under any normal circumstances.

I'm sure these clarifications will make combat more dynamic.

They can only pass through an ally's square without penalty, as long as they don't stop in an occupied square.

Right, you can pass through an ally's square (but not remain there). You cannot pass through an enemy square.

Normally. You can pass through the squares of enemies two or more size categories larger or smaller than you, but you provoke opportunity attacks and you can't end your move there unless you're Tiny. (In fact, Tiny creatures must do this to attack you, as they have Reach Zero and can't hit an adjacent square.) Thus, a Small Halfling Rogue could dodge under the legs of the Large Ogre to get to the other side of him in order to flank, and since the Rogue has a bonus against Opportunity Attacks, it's even a reasonable strategy. (See page 205, Occupied Squares, in the Rules Compendium)

One last clarifying question then: let's say the monster takes its opportunity attack as that characters is trying to move thru its threatening squares. After that immediate reaction, can the character then finish his move action to the extent of his speed? IIRC, a monster or character can only make one opportunity attack per target per round.

Normally an Opportunity Attack (even if it hits) will not interrupt movement - you can continue your movement as normal.

There are some powers and abilities that do stop movement on a successful hit - but those are exceptions, not the rule. The Fighter has Combat Superiority which would stop movement.

Normally characters and creatures only get one OA per round... but there are exceptions there as well (Combat Reflexes, etc).

Some confusion of terminology there. An Opportunity Attack is not an Immediate Action. It is an Opportunity Action. Each character may make one Opportunity Action per Turn, but only one Immediate Action per Round, and you can't take either during your own Turn. Each character has their own individual Turn. A Round is from the beginning of your Turn to the beginning of your next Turn. So, if a whole mess of monsters try to run past you, each of them acts on their own Turn (even if they're identical monsters), and you can take an Opportunity Attack against each of them as they run past.

As a side-note: the other thing I realized re-reading the movement rules: a character can move right thru a monster's space, but will provoke an opportunity attack in doing so (unless the monster is two sizes larger or two sizes smaller than the character). I thought they couldn't pass thru under any normal circumstances.

Thanks for the clarification. I'll explain this to my players next week. I'm sure the rogue will be bummed to learn that he can't waltz past the ogre for a backstab without getting bashed first.

Here's the entry from PH 4e pg. 283 that prompted this comment:

Moving through occupied squares:

Enemy: You normally can't move through an enemy's space unless that enemy is helpless or two size categories larger or smaller than you. Moving into a nonhelpless enemy's space provokes an opportunity attack from that enemy, because you left a square adjacent to the enemy.

I'm probably mis-reading it, but the second part seems to imply that you can move into an enemy's square, get hit with an opportunity attack as the penalty, then proceed on through. But as I re-read it, it looks like you can move thru a 2x/-2x creature's space, but he gets an OA on you.