I agree with Mehitabel- wait to fertilize. Just use water for now. Or you can go to
Welcome to First Rays Orchids and order a bottle of Ray's root growth serum. You add about a teaspoon to a gallon of water and it promotes root growth and I've found it very beneficial when repotting my plants- it's helped them "aclimate" faster...(if that makes any sense) as for the root breakage. I say wait it out...see what they do. If they start to turn dark brown and mushy then they're a loss, but they could still be helpful for the plant at this stage...If an Onc is ailing, they let you know pretty quickly with shriveled pseudobulbs, flopping leaves, and yellowing...
On of the big things I've learned with Orchids is patience. I think growing these plants has really helped me over the years since I was never known for my patience. (look sensei, young grasshopper is learning! ha) Sometimes, it's a wait-and-see game with them...do a little this, wait, do a little that, wait...and if your plant likes it, it will eventually show you. Trust me, if you orchid is happy, you'll know it.
I've been only growing for about 3 years, but during my first year, I killed off 3 plants in rapid succession...I think all of us have toss sad dead plants at one time or another...
Here are some good resources to check out:
AOS | Oncidium (oncidium culture sheet)
AOS | Members Only Area (orchid newbie basics)
First Rays' Free Info (free orchid info)
The more you read and learn, the more confident a grower you'll be. Happy growing! Keep us posted!