I think the care sheet given to you is playing it too safe, maybe for the sake of most newbie orchid growers who oftentimes tend to overwater their orchids eventually killing the plant. But typically for me, the Oncidiums I have grown here in my area, being very dry and such a low humidity location, they are always thirsty here especially during these warm months. I just give a bit more interval in watering when the cold season/winter is here. You will have to adjust your watering regimen as the season/temperature/light level changes. As already mentioned, I just use ordinary tap water, not distilled water. I do my watering early part of the day, not at night.
Growing them will vary from location to location since we have our own micro climates, so you will have to observe closely your plant as it goes, watch the pseudobulbs and the leaves, they will show you if they seem to be thirsty. I would not be surprised if the blooms drop sooner since the plant is still acclimating in your area. Some areas have good humidity levels, so you may have to adjust watering again.
Oncidium leaves are often with brown spots, they just get it a lot, I would not worry much unless it is spreading way too fast, looks sunken, or oozing then something else is going on there. That is why it is important to provide good air circulation all over the plant.
A good indoor/outdoor sensor will be nice to have, so you can really see what your temps and humidity levels are inside your room.
Since you know the name that it belongs to the Brassia genus, these group of orchids are native to Mexico, Central America, West Indies, South America. Imagine the conditions there, very tropical and humid. That should give you an idea what kind of humidity levels this orchid is looking for so it will endure nicely. Hope that helps!
