Many of the new growths on my oncidiums recently turned yellow and soft. It could either be rot or some kind of disease, but I've never seen anything like it The first picture is a Miltonidium Bartley Schwarz "Highlander" and the second is an Oncidium Sharry Baby. Feel free to chime in with any ideas as to what that might be.
The culprit is likely bacterial soft rot. To prevent spread into older growths, you need to cut off any growths showing symptoms of the rot.
Afterwards, cut back on water, especially right after cutting out the diseased growths. Adding calcium to your fertilizer regime may help reduce susceptibility to rot; try 1 tsp gypsum dissolved per gallon of water. Gypsum dissolves slowly, it could take a day or two to dissolve. Add some of this solution to the water you provide you plant a few times each month. Epsom salts provides Mg and prepared the same way, is also beneficial.