Acton is a player new to Competitive, he's played at Regular REL at her LGS with friends and they are pretty flexible with rules application.

Acton is playing Nana at a PTQ. On Acton's second turn, he draws a card, then untaps his land. Nana calls a judge, to which you respond. She explains that Acton drew and then untapped, but since he drew before he untapped he was drawing extra cards, wasn't he? When you ask if Nana had any responses she'd like to insert she says no.

Is this OoOS, is this GPE:DEC, GPE:GRV or something else? Why?

Seems like OoOS to me, but recent discussion on another thread got me to think about this question more.

It doesn't fit DEC because he committed a GRV in not untapping first. Do we assign then a penalty for GRV? It seems like the right game state is reached, but technically a rule has been violated.

What if this is Acton's fifth turn, and he's been drawing before untapping the whole game and Nana just realized it now? Do we hang around the table to keep issuing GRVs until he gets a GL and then follow him around all day and issue GLs until he drops?

Where does the madness stop? :)

Thoughts?

This is pretty clearly OoOS. There's no infraction here.
I'd remind Acton of the order of steps in the Beginning phase and that he
should be untapping before drawing, but honestly, there's nothing to worry
about here, and no possible potential for advantage. (Unless, of course,
Acton is running some odd cards that only optionally untap, but that's not
the case here.)
I'd probably peek in on Acton for a round or two now and again to see if
he's doing things right, but I don't see anything to worry about at all.

This is pretty much ‘the’ cookie-cutter example for OoOS.

No Infraction (unless we suspect him to be cheating by gaining an advantage from drawing the card first, but thats obviously another story), hence no penalty, and educate the player how it is done right.
I'd also tell the player why this is (or could be) important.

Heh, I can imagine phantastic sutuation where Acton could possibly use this for his advantage. If he knew what card is on top of his deck.. And he planned, that, if his opponent would cast something evil, he would need to cast that card from top of his deck in response. And to do that, he needed to draw that card, try to untap, wait for opponent's reaction, and then place a wrong card (that he didn't draw) back to the top of the deck….

But, well, this really is complete madness, and also isn't a GRV or DEC, it's plain straighforward Cheating.
And if it's not Cheating, it sounds pretty much like clear OoOS for me. As the player's new to Competitive, after some questioning it should be clear that it's not Cheating and not an attempt to gain some advantage, so - OoOS, no penalty, remind to the player of order of the steps and ask him to be careful in the future.

I would say that this is nearly universally textbook OoOS.

I use the phrase “nearly universally” becuase there are some annoying cases where players make decisions during the untap step (Vedalken Shackles, for example.) In the Shackles example if a player draws their sixth island when there's a juicy six-power creature under their opponent's control, then suddenly they got too much information too quickly.