We (Girlfriend and I) recently purchased a Town Home. It came with two problems and a great price.
1.) Pet urine
2.) Water heater explosion

At this point you probably are wondering why the heck did you buy it????? It really isn't that bad! Water heater was caught early and was replaced with a much better system. The pet urine however is ridiculous! To think people actually live like that boggles my mind! The carpet was just ruined and the urine actually caused a particle board piece of furniture to adhere to the carpet when they moved there crap out!

Here is the question..... The carpet, molding, casing and effected drywall has all been removed. This yielded the extent of the subfloor damage. The subfloor consists of what appears to be 1/2" to 5/8" plywood with a 1/2" MDF board attached on top. The MDF is part of the original construction from 1968. As the house sits, the plywood was laid upon the joists and then the walls are framed ontop of that. They cut MDF to fit around all wall framing with a 1/4" gap from 2x4's. If I were to bet on this, i would say it was city code problem. The inspector came along and said the subfloor was too thin is my best guess. Thus the laying of the MDF, which the original cabinets and sheet vinyl are laid upon amongst other original things. Our city seems to be specific on subfloor thickness when i asked about code at the city building. Our current setup is just adequate.What baffles my mind is that the water heater leaking from the second floor did not cause the MDF to "puff up" in any water affected area on either level of the town home. But, the pet urine created puffs in some areas and none in others. This whole thing has affected around 8 4'x8' panels of the MDF to varying degrees. As we are on a budget I am unable to replace the entire subfloor. Just replacing the affected areas is the option at this point. It will be very easy and requires very little cutting. My question is, can I replace the affected panels with a slightly higher grade OSB rather than MDF? I can get my hands on OSB at a better price than same thickness MDF (almost half the cost of MDF). Would it fly having patches of OSB amongst the MDF if it is the same thickness? And is it better to screw the panels in rather than nail? And should the screws or nails make it all the way to the joists from the top layer of subfloor?

Once the panels are replaced the entire subfloor is going to get a complete sealing with Zinsser BIN shellac primer as well as every surface that will accept it! I am also going to spray this primer under the replacement panels before installation into the subfloor.

As it is not needed in the majority of the home, we don't want to replace all the MDF. Any help is greatly appreciated.

Thanks in advance....