Fitted OSB board got wet.

[conny]

I've laid some 8' x 4' x 9mm OSB as a sub floor for my new shed, but despite covering it with 2 layers of polythene sheeting as protection against the rain till the shed is constructed, it is showing signs of wet patches.
If I remove the sheeting and allow it to dry out will it dry out without distorting or will it swell up like chipboard and be useless?
I've never used OSB before so I'm not familiar with it's wet resistant properties.

 

[foxhole]

It expands very slightly when wet , but I wouldn’t put any flooring down in a shed until completely watertight.

 

[conny]

Just an update on this. What I thought was a wet area on the OSB turned out to be a patch of condensation between the double layer of polythene sheeting!
Took the polythene off yesterday and the OSB was completely dry. It is now covered with a thick, single layer, tarpaulin with plenty of overhang all round and the edges weighted down with bricks, blocks and lengths of timber. Should keep it perfectly dry until new shed arrives.