You ask a number of sub-questions:
1. Are opportunity attacks considered "on my turn"?
(I've edited the question slightly to be more meaningful)
Opportunity Attacks PHB p.195
To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature.
You also don’t provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.
Reactions PHB p.190
Certain special abilities, spells, and situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else’s.
Answer: They are not considered on your turn unless the opportunity attack occurs on your turn.
Note: the only way a creature can provoke an opportunity attack on your turn is to use its reaction to move, as its movement, action and bonus action are not available outside its own turn. It's not something that is likely to happen often. Also it is common that the abilities that allow movement on a reaction specifically state they don't provoke opportunity attacks (e.g. Relentless Avenger PHB p.88) or are teleportation based (e.g. Misty Escape PHB p.109).
2. Do I get an opportunity attack using the Polearm Mastery feat when a creature enters 15' range because my reach is 15' with Long-Limbed and a reach weapon?
(question edited to be more meaningful)
Long-Limbed VGM p.119
When you make a melee attack on your turn, your reach for it is 5 feet greater than normal.
Answer: No you don't. Your reach is not 15' until you make an attack.
The ability states you get +5' reach "When you make a melee attack". The order of events is: a creature provokes the opportunity attack, then you take the attack. Therefore the condition is not met until you are already attacking them, at which point it makes no difference.
3. Or do I have to wait until they reach 10 feet as Long-Limbed states it is attacks on my turn that I gain the extra 5 feet?
Answer: it is not because it is not your turn, it is because you haven't made a melee attack until after the opportunity attack has been provoked already.