It is almost always best to repot as new roots are beginning to grow. Most orchids only make new roots from the new, developing growths. Some orchids develop the new growth quite a while, and make new roots when almost mature. A few completely mature new growths and even flower, then make roots from those growths.
But older growths rarely make new roots. So if you repot at the wrong time and damage a lot of roots, the plant will struggle to take in adequate water until it can make new roots. All repotting damages roots, even if you are very careful.
Oncidium hybrids usually make new roots around the time new growth develops. The weather is warming up, so there is a good chance your plants will make new growth, and new roots, before long. Once they start making new roots, they will be able to take up more water, and should look better.
You might have cut off healthy roots. This is one reason I almost never cut off roots when repotting. Most of the time I really can't tell what is good and what is bad. Especially with Oncididums I would recommend against this, unless they are obviously black and slimy.
Before your plants begin growing, make sure the roots don't completely dry out. If you're using new, large bark, you might be able to water every 1-2 days. When they begin making new growth be sure they don't dry out. If new leaves get horizontal pleats the plant isn't getting enough water.
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