First, let's all agree that Mesaana is still in the White Tower. Actually, for those who don't know, Sanderson confirmed this in a Q&A:

Q: Is Mesaana still in the Tower?

A: Egwene makes some deductions about this at the end of the book. Egwene is not incorrect.

And, let's agree that she is posing as an AS (I know this is a debatable point, but we need to assume she is for this discussion to have any meaning).

Sanderson also stated the following, about AS in the White Tower:

Every remaining Aes Sedai in the Tower has retaken the Three Oaths.

Note: BS says that all remaing WT AS retook ALL 3 OATHS, so either Mesaana is currently bound by all 3, or by none. More on this below.

So, let's agree that this statement includes Mesaana, so that Mesaana at least appeared to retake the 3 Oaths during the process in which all the remaining WT AS retook them.

The question I want to address is ... is Mesaana bound by the 3 Oaths?

Now, I've seen it suggested that Mesaana IS bound by the 3 Oaths, and that she was able to answer "Are you a darkfriend" with "No", because she doesn't consider herself to be a lowly DF, she considers herself to be a Chosen, which she truly views as a legitmate distinction.

Sanderson also has said some things on this particular issue:

Q: Do the Forsaken consider themselves Darkfriends?

A: Oooh, you are tricky! I know what you’re doing. [For those that don't, this guy is trying to figure out if this is how Mesaana beat the Oath Rod. - Terez] I would say that, in general, yes they do consider themselves Darkfriends, though there are ways somebody would be able to get around that. I would say, yes, that that is not the sort of mental gymnastics that…it is very easy to convince yourself that you are not a Darkfriend.

Q: [Messana] could swear that she’s not a Darkfriend on the Oath Rod, right?

A: As long as she believed it to be true. Every remaining Aes Sedai in the Tower has retaken the Three Oaths. You should be thinking about ways to defeat the Oath Rod. There is a way to do it.

However, based on the bolded part of BS's answer above, even if Mesaana could have replied "No" to the DF question while under the do-not-lie Oath (which BS says may have been possible), I don't think that is what happened.

The key here is, BS says that there is a way to defeat the Oath Rod, and (as I interpret his response) that we should focus on that issue, with respect to the Mesaana question, not on the issue of whether she could simply have replied "No" to the DF question while under the do-not-lie Oath.

Which I take to mean ... Mesaana is not currently bound by ANY of the 3 Oaths. This makes sense, actually, because regardless of the do-not-lie Oath and whether she could have answered "No" to the DF question, Mesaana would never have let herself be bound not to use the OP as a weapon.

Therefore, as I see it, there are 2 possibilites:

1) The 3 Oaths "activated" when Mesaana swore them, but she knew a way to deactivate them, and did so before she answered the question about being a DF.

2) The 3 Oaths never activated when Mesaana swore them. She knew a way to keep the Oaths from activating.

And my guess is, 2) is almost certainly the correct answer. If 1) were possible, Binders/Oath Rods would have been useless in the AoL - criminal channelers (like, say, Semirhage) could have sworn any Oath they were forced to swear, and then deactivate it/them anytime thereafter, unbeknownst to anyone else.

Whereas with 2), the AoL AS who were putting the Binder on a criminal channeler could (and certainly would) ensure that the criminal wasn't trying any tricks to defeat the Oath Rod during the process, to ensure that the Oaths actually activated. Whereas, the 3rd Age AS don't know about any tricks to defeat the Oath Rod, so they don't know to look for one, and Mesaana could get away with it.

So the question is, what sort of trick did Mesaana use to prevent the Oaths from activating?

I saw this discussed very briefly in another post, and in that post someone (LitN, I think) suggested that Mesaana put some sort of Keeper weave around herself, which prevented the Oaths from activating on her (the idea being that nothing external can pass through a Keeper weave to affect whatever is behind it).

This is an interesting idea - Mesaana could have inverted the Keeper weave, so that no one knew it was there. So this could be the answer (or something similar to this - some sort of invisible "shield weave" that prevented the Oaths from activating).

But, we don't know for sure that a Keeper or any other type of shield-weave would actually prevent the Oaths from activating. It's just speculation

Another possibility I was thinking of is ... I'm pretty sure we know for certain that a Binder only works on channelers. So, maybe Mesaana knows and utilized a weave that essentially made herself a non-channeler during the time she took the Oaths, so that nothing actually happened when she took them. We know the Forsaken know of tricks that make themselves appear to be non-channelers (e.g. Lanfear must have done so when she masqueraded as Keile Shaogi), so maybe this idea isn't too much of a stretch.

In this case, we know the idea would work if such a weave is possible, since we know Binders only work on channelers.

Anyone else have any ideas on what Mesaana could have done?

I'm guessing that she couldn't have, say, subtley diverted the flow of Spirit that was channeled into the Oath Rod to activate it - that would have been noticeable to others, I would think.