look at the pattern of the nails to see where the joists are. If you lay a piece of 2x4 or something to span at least two (preferably three) of them, and the bath feet on it, that will take the spotload off the flooring. The bath feet will have some adjustment in them to go higher and lower that should take up the thickness of your support. (remember to have the bottom of the bath tilting slightly towards the plughole)
Even a slab of 18mm ply would do, resting on the joists.
Chipboard is particularly weak and unsuitable for rooms that contain water.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local