22mm OSB3 vs Ply bathroom subfloor

[DIYWarrior]

Just sanity checking if 22mm OSB3 T&G is fine for a subfloor 1st floor bathroom instead of Ply? The joists are 400mm centres and I plan to stiffing up with noggings. I'm replacing the existing 22mm floorboards which are in bits and have been cut in lots of places by previous owners. It's an ensuite so room is small enough (~2.4M x 2.4M) The floor will be tiled and I was planning on the 22mm T&G OSB3 with a decoupling mat on top.

A plumber who I had doing some other work reckons lots of the ply supplied nowadays has cheaper glue used in it and is more prone to delamination. He prefers to overboard the existing floor where possible with a 6mm magnesium board.

 

[Deluks]

You can get structural cement boards that won't swell or delaminate. Use a decoupling mat over this.
Otherwise I would go 18mm ply with 6 mm cement board on top.