Does when a fighter's mark get applied permit this Tactical Superiority combo?

Question

I read through When does marking occur?, but I'm still not sure that the sequence of events actually happens the way I'm hoping it happens for the following combo to work.

I'm a level 16 heavy blade polearm fighter with a Glaive, and I've got the usual feats like polearm gamble, polearm momentum, etc.

I'm looking at the feat Tactical Superiority which says:

Whenever you hit an enemy marked by you with an OA, that enemy is slowed until the end of your next turn.

I'm not sure when the mark applies during the following attack sequence: Say I'm standing in a square, and an enemy enters a square adjacent to me, triggering my Polearm Gamble, so I attack it with an OA. Could I mark him as I'm attacking, triggering the Tactical Superiority feat, slowing him until the end of my next turn? Or does the creature have to be already marked by me before entering my adjacent square?

In short, a mark says it can be applied whenever you attack, whether you hit or miss, but can you mark as part of the attack?

Answer 1

That combo appears to work by RAW.

From the PHB, p76 (emphasis mine):

Combat Challenge
... Every time you attack an enemy, whether the attack hits or misses, you can choose to mark that target. ...

Every time you attack an enemy, you get to mark them.

From MP2, p143 (emphasis mine):

Tactical Superiority
Whenever you hit an enemy marked by you with an opportunity attack...

Tactical Superiority doesn't trigger until you actually hit the target.

From the RC, p92 (emphasis mine):

Sequence
The order of entries in a power description is a general guide to the sequence in which the power's effects occur. For instance, an "Effect" entry might appear before an "Attack" entry to show that something happens before the attack.

This section of the rules makes it clear that the "Attack" part of the OA happens before the "Hit" portion.

Based on those three quotes, it seems pretty straightforward to say that when you make the attack roll against the target you get to mark it, so when you actually resolve the effects of a hit (including Tactical Superiority) the target is already marked by you.

That said, a counter-argument could be made that the "whether the attack hits or misses" section of Combat Challenge is meant to imply that the mark is applied after the attack is fully resolved. Any time you get down to things like this that the rules sort of imply but don't state outright you're going to have to ask your DM for a ruling.

I personally would probably allow this combo to work; fighters already stop the creature's movement on OAs thanks to their Combat Superiority class feature, so slowing them on top of that is unlikely to make much of a difference most of the time. If you or other party members have some additional combos to really take advantage of the slowed condition then I might reconsider allowing it, but on its own it's not very powerful, maybe not even worth the feat for Tactical Superiority.