Forgot to mention that it's gotten down to 53 once or twice over the last week at night, 78 to 82 during the day. Could low temps be part of it?

It might be the potting material. If it has started to break down it must be replaced. One of the reasons I use lava rock. It won't break down. It does require me to water daily and use terra cotta pots. If it drys between watering then your watering is good. Give details on your potting material. Age, type, condition....picture

It's mostly pine bark, with some perlite(20%) and lfs(5%)mixed in. May have some of better grows orchid mix mixed in. Very loose and airy. I potted the big one up in mid February 2016 - the smaller one in December 2015. I've since switched to using inorganic clay pellets or mounting for most orchids. This is one of my last still in organic media.

I do not notice any new growth. Is there some new growth? Is it hiden?

There are two types of leaves: tru leaves and brachts. Tru leaves are ones on the top of the bulb. All others should get yellow and fall off. When true leaf is yellow, this is sign of trouble, whatever it is. Often it is stress.

One of them has a new growth. Both have been flowering like mad - spikes went up a couple of months ago and flowers have just recently opened. One spike on the smaller one, three on the big pot. All the yellow leaves are on the sides of the psuedobulbs, not the leaves on top.

I get yellow leaves fairly often on the oncidiums and figure they're probably the older leaves. All plants eventually shed their oldest leaves, don't they?

Maybe it is resting after a lot of flowers and sheding some old leaves. Reduce water for a while. Let it rest...

If they are side leaves, so no worries, provided if there is no water between side leaves and pseudobulb. By the way, the reason why oncidiums' leaves are looking bad, is a water between small leaves on new young growth! This is something many ppl are not paying attention.

I agree with vtandrea, old leaves. Let the plant rest over the winter.

Jane