I remember it from grade school in the early 80s (northeast PA) and same stuff: a somewhat thinner, harder version of the stuff manila folders are made of.
We used it in art class a lot because it was stiffer than regular paper, especially for a base to glue stuff onto.
One teacher, when another kid asked her why it was called oaktag, said it was because the paper was made from oak wood. I don’t know if that’s true or if that was just her “sounds right so it shuts 'em up” response.
When I first read the thread title, I had no idea, but when you mentioned the poster board, it sort of rang a bell. Maybe I had a teacher use the term way back in elementary school…
Last year’s Newberry winner, When You Reach Me, has the main character carrying an oak tag project to school (in New York in the 70’s). That’s the first I’ve ever heard the term.
Tie a yellow ribbon 'round the old oak tree, wasn’t it?
It’s “oak tag” and not “oaktag”?
Familiar with it from elementary school, SE Penna, early '60s. Refers to stuff similar to file folder paper only – stiffish, buff-colored.
Interesting. And thanks for the reminder about this book–I’m reading it for class next week and still need to pick up a copy!
At first I was going to say “no”, but then it dimly tickled a memory from elementary school and the image of “something like poster board, but not quite exactly the same, for pasting a collage onto for the visual aid for a school report” popped into my head. Which is apparently what you are referring to. So I guess my actual answer is “yes”.
(Grew up in the mid- to late 1970s in NYC.)
ETA: That was before reading any of the replies including mention of “When You Reach Me”, which I will now look up.
37, Illinois, never heard of it. Heard “tag board” very infrequently (and probably wouldn’t have guessed it was poster board) but never “oak tag”.
OMG, a trip down memory lane fer sure. And bring on the mimeo!!! 
Grade school in the 60s in northern NJ. Ah yes, I remember oaktag very clearly. More than a few projects were done on it in my youth…
In NY in the '60s it was the only stuff we used. Never heard the term poster board, only oak tag, and it might well have been used generically.
Never heard of it. 32, grew up in California.
I’ll be dog, I remember hearing ‘tag board’ as an alternative to ‘poster board’ but I’d never heard ‘oak tag’ before today.
Texan here and I’ve never heard “oak tag” either. Tag board, yes though.
Yes, we used oak tag very often back in the 50s-60s. But I haven’t heard of it in a long, long time.
NYC, 70s. Never heard poster board - it was always oaktag.
50’s, Ohio…have heard both oak tag and tag board,but not for years.
21, New Jersey, have always known it as a synonym for posterboard.
ETA: My brother (17) has never heard of it.
OMG it’s like you put me in a time machine and sent me back to the mid 50s. Yup, we used oak tag in elementary school. I turned out to be an artist so I fell in love with a lot of difference kinds of papers, pencils, inks, paints, etc.
I’ll return the favor. How about ONION SKIN???
i haven’t heard of it for almost 50 years! Thanks for the memories!
I am a teacher and I have oak tag in my classroom at this very minute! But it’s not nearly as nice as posterboard or other paper because of being all slick and yellowy.
For protest signs I recommend the large paint stirring sticks, free at Home Depot.