I had the same question, so went to ask Mr. Google who says that OSB is "Oriented strand board is a type of engineered wood similar to particle board, formed by adding adhesives and then compressing layers of wood strands in specific orientations."

Oriented Strand Board

FWIW there are large cutting mats of the size you need for your table. I converted an old banquet table as my main work and cutting surface using a MegaMat that fit my tabletop. Even better, you can cut yours to fit the mat size of your choice.

These mats are surprisingly affordable, and durable. I have used mine now for about 6 years and hard...it is still going strong. I would be lost without this large work surface that is safe to cut on.

https://quiltersrule.com/ProductThumbs.php?category=15

My husband got a thick piece of plywood for me, we put batting on it, and then a heavier cotton cloth. Both the batting and cloth were wrapped it to the backside and stapled it down. The cloth was a cotton, but heavier like sailcloth or a painters dropcloth. I've had it for years now...still in good condition.

If you'll be ironing on it, cover the wood with heavy duty aluminium foil so steam from the iron doesn't warp the wood. OSB is prone to warping from water/steam. I've used foil then a layer of batting then ironing board material for the top of my wood and it works wonders. one layer of batting is soft but still will allow you to put a cutting mat on top and cut easily so you have a two-for-one surface.