As a general rule, "An opportunity attack is a melee basic attack" (Compendium glossary), so this doesn't come up much.

However, there are special cases: Ninth Legion Student [Lesser Style] lets you use Direct the Strike (ranged 5) "in place of a melee basic attack when making an opportunity attack."

Now, both the Glossary entry for Ranged Attack and the entry for Opportunity Attack explicitly say that ranged powers provoke OAs from adjacent enemies. To me, this means that ranged OAs provoke OAs unless a feat/power/item/feature explicitly lists an exception.

Conclusion: YES. Ranged attacks always provoke opportunity attacks, even when the ranged attack is itself an opportunity attack. It's really uncommon though, as using ranged attacks for OAs is a rare exception to the "only-MBAs-for-OAs" rule, and there'd have to be a second enemy adjacent to you to take advantage of the granted OA anyway. [Note: OAs interrupt the action that provoked them, so causality might get messy to track if this procs a chain of OAs.]

No, Invisible Creatures Do Not Provoke Opportunity Attacks

(From Enemies That Can't See Them)

From page 221 of the Rules Compendium:

An invisible creature can take advantage of several benefits.

• It can't be seen by normal forms of vision.
• It has total concealment against any enemy that can't see it.
• It has combat advantage against any enemy that can't see it (but it still has to be able to see the enemy).
• It doesn't provoke opportunity attacks from enemies that can't see it.

(Emphasis mine)

While it is true that attacking will cause a creature to no longer be hidden, it will still retain the benefits of being hidden (including not provoking an opportunity attack) until the attack resolves. On page 153:

Not Remaining Hidden: If the creature takes an action that causes it not to remain hidden, the creature retains the benefits of being hidden, such as combat advantage, until the action is resolved. The creature can't become hidden again as part of that same action.

As defined on pages 196-197, opportunity actions (such as the opportunity attack) are interrupts, and therefore occur before their triggering action resolves:

Interrupt: Just like an immediate interrupt, an opportunity action interrupts its trigger, taking place before the trigger finishes.

Putting all of this together, you can see that the opportunity attack would not be provoked by an invisible creature making a ranged attack against an adjacent creature. The invisible creature's "doesn't provoke opportunity attacks" benefit would not be lost until the attack resolves, but an opportunity attack would have to occur prior to that attack resolving, and therefore the opportunity attack never occurs.

To break it down into a sort of sequence of situational facts/events:

1. Creature A is invisible to Creature B, and they are adjacent to each other.
2. Creature A uses a ranged attack against Creature B, becoming un-hidden as a result of making an attack action.
3. Ordinarily, this is when an opportunity attack would be triggered from Creature B, but since Creature A has the benefits of invisibility against Creature B, the opportunity attack does not occur.
4. Creature A's attack resolves.
5. Creature A's invisibility benefits are now lost.

So, at the time that the opportunity attack would ordinarily trigger, the attacking creature still has the benefits of invisibility and no opportunity attack can trigger.