Is MgO board waterproof?

No, MgO board is not waterproof but it is water resistant.

No, MgO board is not waterproof but it is water resistant. MAXTERRA MgO panels do not rapidly deteriorate when immersed in water over a short period — as happens in floods and extreme rainfall events — and can maintain structural stability even under the force of minor bending. High-performance MAXTERRA MgO panels do not warp, swell, or delaminate. They are not fully waterproof but compared to most wood and all gypsum drywall products they are far more resilient when facing temporary water inundation.