I was hoping to have plywood sub flooring underneath the carpet but NO ... it is OSB (20 yrs old). The original plan was to use paving paint to paint the 2 bedroom floors including stairs due to non existent budget ..Can I paint OSB flooring or am I better off putting, cheap thin plywood sheeting ?
You can paint the OSB board, but there is some preparation required. First, you need to clean it with Trisodium phosphate to remove the waxy layer applied to its surface. An all-purpose cleaner and paint prep like Tricleanium will do this job for you. Once you've cleaned the boards and allowed them to dry thoroughly, you can apply Feast Watson Timber Floor Paint.
If the OSB has started to degrade, then you might like to prep it after cleaning with Feast Watson Proofseal before painting.
Please be advised that these products are designed by the manufacturer for solid timber flooring, not for a particle OSB board. You might wish to do a test section in an inconspicuous location to verify if they'll work for you.
If these products don't fall within your budget, I recommend you use the most budget-friendly oil-based enamel paint you can find. Something like Taubmans Tradex in a 10L would be reasonably budget-friendly. However, it's not explicitly designed to be used on floors, but it is a hard-wearing paint. Exterior paints like paving paint are best left for outdoor purposes.