My daughter knew what Oak tag was when I asked. They use it in class. So still used in Central Jersey.
I grew up in NJ, and am now in my late twenties - we used the term all the time. It seems to be a NY-metro-area regionalism.
For those who remember mimeo, was it just the kids in my school who referred to mimeos as “dittos” and the carbon paper forms used to make them as “ditto paper?” We loved getting a chance to make a ditto.
Onion skin is that very very thin, nearly transparent, yet surprisingly strong paper that many fine Bibles are printed on.
At some recent mass-scale protests (like the G8/G20 protests) some folks have been stopped before they ever get to the protestor corrals because their paint stick sign holders are seen as potential weapons. Wrapping paper tubes are too lightweight to hurt anyone. And as a bonus, you can cut a slot in it and slide it over your hand, which makes your sign easier to carry.
Never heard the expression “oak tag”. Southern Ontario. I have heard the expression “onion-skin (paper)” though. How many of you know what I mean when I say “duotang”? 
Depends entirely on what you’re doing with it. One of its common uses is as industrial sewing pattern paper. It’s heavy weight, so it doesn’t fall apart with use, and it’s got a much nicer hard edge than poster board does. Industrial sewing either puts the pattern directly on the fabric, and the cutter traces the pattern onto the fabric with a marker or grease pencil, or it’s put on a piece of regular paper the width of the goods to be cut, and it’s traced on to that. That paper (called a marker) is then stapled (in the wasted bits) to a stack of fabric (can be a foot thick and 100 feet long, in a big factory) and the pieces cut out. Either way, it’s important that the edge hold up, sometimes for years. Poster board and like stuff doesn’t hold up; the edges fall apart when it’s cut out, even if it’s not used.
The nicest stuff used for patterns has one side manilla colored, and the other greenish, which makes it really obvious if a piece has been put down upside down. That’s very rare these days, largely because there’s no demand for it, and it’s more expensive. (The place I buy mine from says they can get it, but it would have to be special ordered, and it costs three times as much.) Most pattern making is done on CAD systems, and most of them can do marker making, which can then be printed to a plotter. (And, of course, lots of the factories are no longer in the US.)
Just FYI, vorpal, this thread is over 2 years old, and **xoferew **hasn’t logged on since July 2011. So don’t hold your breath for a response from him/her.
I’m guessing you found us via google search? Welcome to the SDMB. Pull up a thread, make yourself comfortable.
I heard it a lot when I was a child. I am 49 now. Upstate NY.
Southerner, never heard this phrase.
I know I’m responding to a two year old post, but not a New York regionalism. I heard of it growing up in Western Pennsylvania, not far from Pittsburgh.
Eastern Ontario here. We said “oak tag”, “onion skin” (although we were probably as likely to say “tracing paper”), and “duotang”.
I’m 53 (a few seconds away from 54) and spent all but 6th grade of my elementary school years in either CA or WA. I don’t remember ever hearing that term. mmmmm, I can still smell dittos though. 
Ditto; we called it tag board, but I know what oak tag is, probably from reading. I doubt anyone I grew up with would know what it is. We didn’t use that term.
I don’t know where the “oak” part of the name comes from but the “tag” part is because it’s used to make tags, you know, like these: Shipping Tags
Tag stock, as it’s also called, is used for file folders and dividers, too. It’s a tougher than poster board, which is a nicely finished, rather soft cardboard. It can come in colors, but you usually see the off-white manila flavor. Used it in school in New England in the '60s. Used it in a printing job shop later.
Heard of it long ago, I think going to elementary school in the DC area. I don’t recall the term ‘poster board’ being quite so generic at the time. It would be called cardboard, heavy stock, some brand name I’m not remembering now, probably a few other things. I think oak tag would be at the bottom of the list that I encountered.
Never heard of oak tag or tag board. Grew up in eastern upstate NY.
There is a song, it’s called Tie A Yellow Ribbon Around the Old Oak Tree 
From NH originally and grew up calling it Oak Tag. Recently I moved south and instead of saying Poster Board, Oak Tag slipped out. No one has ANY idea what I was talking about. Just to prove I wasn’t crazy, I Googled Oak Tag … to back up my “northern” word.
I see several NJ/metropolitan area people here who said it’s very familiar to them, but I was born and raised in NJ 45 minutes from midtown Manhattan, and I never heard or read of “oak tag” or “tag board” before I saw this thread.
If it’s a regionalism, it’s not really that equally spread around the region.
When I saw the title of this post, I thought, “I think I know what this is.” I was wrong.
In many areas of the South, the term “pine tags” is used for fallen pine needles. I guessed that “oak tag” meant some sort of oak leaves. It’s a good thing nobody sent me for oak tag.
Yes but only in elementary school art class 30 years ago.
ETA: This was in Northern NJ in case that matters.