When you repotted it, what did the roots look like? I bet what is going on here is that all or most of the roots are dead, and the plant can no longer take up moisture, so it has to use the water stored in the pseudobulbs, and therefore they shrivel. It can sometimes be difficult to tell just by looking if Oncidium roots are dead or alive, but if they were brown and felt hollow, and the outer layer peels off easily, then they are dead.
In the event that the roots are dead, it will just need time. New roots will come with new growth. Keep the plant warm and humid, and keep moisture around the base of the plant (the sphag potting mix you appear to be using will take care of that).
In my experience, Oncidiums with damaged root systems will sometimes recover, and sometimes not, and there doesn't always appear to be a reason why or why not. Have patience, as this can sometimes take a long time, but when you see new growth, you can be pretty sure the plant will survive.
Also a note about shriveled pseudobulbs: Once the plant has new roots, the pseudobulbs will reabsorb some of the water they lost, and will fill back out a little, but they will never be round and plump and smooth like they were before the roots died.
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