Welcome aboard Nik! Oncidiums just do that. The older pbulbs will often wrinkle and turn brown. I have never found it to actually harm the plant though. If pbulbs turn soft and oozy, sometimes I remove them, because I don't want an infection to spread. More often, though, they just turn leathery and hard, and that is OK. You haven't had your plant long, so I suspect, looking at your pot, that it may appreciate a deep watering, and maybe a repotting after it is done blooming. To judge the health of your oncidium, look at how the newest growths are doing.