Within Reach Opportunity Attacks
There is such a thing as being locked into a duel. You aren't so willing to just let a gang of thugs surround you. You don't just stand slack-jawed as a swarm engulfs you. You don't necessarily just need to attack when a creature's back is turned from you, running. You make an attack.
Within Reach Opportunity Attacks
Moving within a creature's reach provokes opportunity attacks, like exiting a creature's reach, getting up from prone while within reach are also considered such movement.
Entering a creature's reach for the first time doesn't provoke an opportunity attack, unless the creature has an ability like the Polearm Master Feat or similar abilities. (like MToF: Boneclaw pg. 121 Deadly Reach)
Remember that you can Disengage to not provoke Opportunity Attacks (unless the creature has Sentinel or a similar ability).
Entering Reach
Entering a Creature's Reach does not Provoke Opportunity Attack, unless the creature is sporting an ability like Polearm Master:
[While you are wielding a glaive, halberd, pike, quarterstaff, or spear, other creatures provoke an opportunity attack from you when you enter the reach you have with that weapon]
Regular Opportunity Attack
A creature can make an Opportunity Attack whenever a hostile creature moves out of their reach. Remember that it costs their Reaction and the attack occurs right before the creature leaves their reach.
Entered Reach, Moved Within
A creature provokes a Within Reach Opportunity Attack, after moving within the Hostile Creature's reach after entering it. Positioning is everything, and striking an enemy maneuvering around you is pivotal to survival.
Long Reach, Moving Closer
A creature provokes Within Reach Opportunity Attacks when they approach you for even closer engagement. This includes not only entering a space closer to you, but entering your space as well, such as when a creature has the Swarm Trait that allows them to stay in the same space as you.
Note Entering Reach above, the creature must have already been within your Reach to provoke any Within Reach Opportunity attacks.
Long Reach, Moving Through
In this instance, a creature provokes multiple Within Reach Opportunity attacks by moving through multiple squares the Long-Reached Creature has Reach of.
Do Note, that a creature can only use One Reaction to make an Attack of Opportunity unless they have an ability to have more Reactions.
