That area has had moisture on it---maybe a pet, or a large plant where water spilled. The spill area is rather large for a pet stain. It looks as if there was some kind of container where the black spots are. That could be mold, but cleanable. Carefully dampen the area with a bleach/water spray and wipe up the stuff.

That is not OSB, it looks more like some kind of hardboard.In any case, unless the area is spongy, there does not look like there is damage other than the stain.

If it smells of animal stain, Nature's Miracle(available at pet stores) will completely eradicate the smell.

There are specific mold and mildew killers available at the hardware store, I would use one of those on this area and let it dry thoroughly. Or use something longer lasting that will keep mold from returning, like tea tree oil or borax. Bleach works well on non-porous surfaces, but for wood I would want something that has lingering toxicity to mold so that if any spores survive in the wood they die when they come to the surface.

Thanks for the suggestions. It's good to know that it's treatable. It was in a laundry room, so I suspect it was water damage caused by a leaking washer or something

As for the material itself, I've included another picture of a normal section of the floor. From looking at pictures of flooring material on the net, the closest thing to me was OSB.. Is there a way to determine the exact material? Since I'm planning to put a floating floor on top, I wanted to make sure I'm doing it properly given the underlayment material.

Thanks again.

Ok, it does look like OSB, must have some kind of coating on it---OSB is usually not white.

Don't waste time worrying about it. The safe approach is to cut out the damaged portion and replace it.

Hmm.. cutting out and replacing seems like a lot of work. Any other opinions? I'm leaning towards replacing since my girlfriend had issues with mold sensitivity in the past.

I can't really tell what the condition of the subfloor is from a photo but it is possible for even dead mold spores to cause problems for people who are sensitive to them. You only have one sure option: replace the material and put a pan under the washer to catch overflow water.

For more information you can buy NCARB's "Mold and Moisture Prevention" but it would be quicker and cheaper to replace the damaged material.

Here is a link that might be useful: NCARB Mold and Moisture Prevention

Just as a follow-up, I ended up wiping/scouring the mold with bleach and water, then sanding the surface, and painting over with Kilz Premium. We're planning to put 1/2" plywood over the OSB and vinyl sheet flooring on top of that, so hopefully there will not be any moisture issues in the subfloor even if we have a washer leak.

Anyway, I know the preferred method would've been to cut out and replace the molded area, but just hoping this shortcut works.

kilz will do you no good what so ever
you need BIN and nothing else

not sure on the mold. If cat urine I doubt any chemical will get the smell out. Many say to use Nature's Miracle....many others claim the smell is still there after using it. When we moved into our home we had pet 'smells'. We decided to recarpet the house. After ripping the carpet out you could see the spots...and smell them. I coated them with oil based kilz. After the odor from the Kilz disapated, the smell from the urine was gone as well.

Replacing it would have been the short cut.

Mold can trigger allergies. Cats can trigger allergies. Mold is no more dangerous than cats. Water leaks, however, need to be repaired. In this case it sounds like you did a great job and as long as there is no more leak, you should be fine.