Tukukkk

The Polearm Master feat (PHB, p. 168) says:

While you are wielding a glaive, halberd, pike, quarterstaff or spear, other creatures provoke an opportunity attack from you when they enter the reach you have with that weapon.

The War Caster feat (PHB, p. 170) says:

When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

Assume that I'm a wizard, with both Polearm Master and War Caster, and I'm holding a staff of power (it's a magic quarterstaff).

Can I cast a spell (with a casting time of 1 action) as a reaction when a creature enters my quarterstaff's reach, targeting only that creature?

This works fine by RAW

There's no reason this wouldn't work, and generally meets the intent of both feats. Running through the checklist though:

1. Are you wielding a glaive, halberd, pike, quarterstaff, or spear? Yes. (the staff of power is wielded as a "magic quarterstaff")


2. Has a creature entered your range with that weapon, provoking an opportunity attack? Yes. (note, it doesn't say you need to use that weapon in the attack)


3. Do you have an opportunity attack with which to substitute a reaction to cast a spell instead? Yes. (per #2)


4. Does the spell you're casting cost 1 action, and target only that creature? If so, then Yes.

So long as all the criteria above are met, then it works as expected.

However, by RAI...

The answer provided by rpeinhardt cites authoritative clarification that this isn't intended to be the case:

Q: [I]f I have the war caster feat and a reach weapon can I use a spell instead of the weapon if they move to 15 ft?

A: The intent is that any OA triggered because you're wielding a
polearm is then made with that polearm.

At this point, it's up to the DM with how to rule here.

Rules as Written: Yes. Rules as Intended: No

Per the lead rules designer, Jeremy Crawford:

Q: [I]f I have the war caster feat and a reach weapon can I use a spell instead of the weapon if they move to 15 ft?

A: The intent is that any OA triggered because you're wielding a
polearm is then made with that polearm

This is Rules as Intended. However, Rules as Written, the literal interpretation allows you to do as you say.