New orchid with dead roots and a fungus
 

I recently got a new orchid which was originally potted in soil. I believe this is an oncidium. I've only ever owned phals so really wasn't sure what roots were alive/dead. They all felt a bit mushy. I re-potted it in bark and its been in the bark for about 5 days when today, I decided to check on it because it looked dehydrated even though I just watered it. I saw a blue looking fungus on it. This orchid's flowers are beautiful, it would be a shame to lose it.
Pictures:
Imgur: The magic of the Internet
Imgur: The magic of the Internet
Imgur: The magic of the Internet

That is indeed an oncidium. I would think that re-potting it out of the soil was a good idea (did you cut off the flower spike?).

Otherwise, I think its going to just need some time to adapt. To avoid putting undue stress on it, don't remove it from its medium just to check on the roots.

It still has the spike. Should I do nothing with the fungus/mold on it. I was thinking of spraying it with hydrogen peroxide 3%?

Personally, I don't have much experience with sterilizing roots with H2O2.

You're welcome to try it, but if I were you, and your primary concern were preserving the orchid for the long haul, I would definitely cut off the flower spike so the plant can refocus its energy on growing new roots and adapting to its new medium.

The spike will do no harm. Leave it and enjoy it. Now that it is in bark, where it has a chance to dry out between waterings, I don't think you have to worry about fungus. But not too dry... these like to stay a good bit more damp than Phals. (If the plant were still out of the pot, I'd say to give a good rinse, but if it is already repotted, don't take it out again, wouldn't recommend the peroxide. The plant looks very healthy, and considering where they were, the roots even look pretty decent. I think you caught it in time. Well done!

The fungus appeared after I put it in the bark, should I use new bark or keep it? Also I'm wondering if I could stimulate new root growth by keeping the top of the bark humid.

Humidity around the base of the plant can encourage new roots, so misting the surface of the bark to keep it moist may help.

However, that being said, you may not see any new roots at all until you see a new growth. If all of the roots are dead, and it looks like they might be, the pseudobulbs will shrivel quite a bit until the new growth puts out some roots. Some Oncidiums won't start a new growth until after they bloom, so if all of the roots are indeed dead, I would recommend cutting the spike to hasten the initiation of new growth. The flowers won't last as long as they would on a healthy plant anyway. It might make it even if you don't cut the spike, but if it was my plant and I was really enthusiastic about saving it, I would remove the spike.

Roberta said the roots looked decent, how do I know if the roots are dead?

If they are hollow and if you can easily peel away the outer layer, leaving behind a wire-like core, then they are definitely dead.

so I'll try to cut off all the dead roots. Do should I do anything about the green fuzzy stuff growing on the roots?