Is this photo site also the plant's actual regular location? (Don't see any window at all so wondered.)
Your temps in the window area is in the 40's? That could be a big issue right there. Most members of the Oncidium Alliance do NOT like temps that cold.
Next, it's hard to tell without a better photo of the pseudobulbs, but your plant may be suffering from desiccation. Generally speaking (as there are exceptions), the finer/thinner the roots are, the more moisture retentive the media should be. In the case of a mounted plant, this translates into watering more frequently and/or using a pad of sphag, coir fibers, or other moisture retentive material on the mount to extend the length of time the roots stay moist. Said pad of material can be placed on the mount with the orchid then mounted directly on top of the material or the pad can be placed so it covers a substantial quantity of roots.