Although mould is a common problem in many houses, it can be potentially dangerous. It's best to remove mould in its entirety once you discover it. Because you must ensure safe and complete removal, the process takes time and patience. In drastic cases, it may be best to remove the plywood from your home altogether; however, in most situations, you can clean it with the proper supplies.
Seal off all vents in the area where the mouldy plywood is located with plastic sheets. This contains the area and prevents the mould from spreading. If the plywood is not a part of a building structure and you can move it, take it outside into the open air.
Wipe away any surface mould on the plywood with a dry rag. If you need to remove mould from a large area of plywood, replace your rag regularly. Never rinse and re-use the same rag, as this spreads mould spores.
Apply a biocide or anti-microbial to the entire surface of the plywood. These types of cleaners seep into the various layers of plywood and effectively kill the mould spores. Wait for the biocide to dry completely.
Allow the plywood to sit for several days to see if the mould returns. If it does, repeat this process. If it doesn't, proceed to the next step.
Seal the plywood with a Shellac. If possible, choose a product that contains an anti-fungicide. You can also purchase an anti-fungicide and add it to your sealant/primer.