but the leaves are turning a weird copper color now (green to yellow to copper) ! could it be related to the spots?

It is normal for the oldest leaves to fade and die. If you are losing many leaves including younger ones than that is a problem. Is it many leaves or just a few?

Sorry for the delayed response shavedmonkey, but I wanted to show you pictures with more prominent spots to help you identify the problem better!

Here are photos from today:

^This one dropped today...
The photos are all of different leaves, and there's more! Here is a link of all the photos.

Also, I've noticed that a flower on one of my den-phals is wilting. It's only been a week since it has bloomed... And the other one has green spots on the back of its petals. I wonder if they caught anything from the oncidium, the leaves look fine though:
The one with spots on back of petal:
The one with wilting flower (petal is translucent?):


Sorry for the horrible image quality. I hope you'll be able to help :(

You need to give more info. Where do you live? Where are these plants, outdoors, indoors. What type of light? How are you watering? What are they planted in?

Are you in zone 11?

Looks offhand that you might be watering the Phals to much, maybe too much heat/sun. The Oncidium looks okay, although I can't tell from your photos. Are the new leaves (baby leaves) turning yellow? Oncids typically have spots on their leaves, freckles. Maybe your plants are getting too much sun??

You need to fill in the missing pieces so we can help.

Jane

As Jane says those spots on the Oncidium leaves are often seen on plants subject to high light and old leaves yellow and die.

If younger leaves are dying, there is probably a problem with your growing conditions.

Overwatering?

Overpotting? Too large a pot for the plant?

Wrong mix?

You are trying to copy the conditions in nature where the ancestors of your orchid grew/grow on the branches of a tree or on a rock. Many new growers overwater and the potting mix ends up being a swamp.

@jane_ny and @arthurm2015, I've tried to mention everything in the description section of my question (check notes). I apologize for the lack of some important points.

My zone: 11.

ONCIDIUM:

So, when I got the oncidium, it had small dark spots on the older leaves, mostly around the tips.

I placed it on a west-window, getting sunlight through glass and curtain. The leaves had turned a dark green, which is why I'd exposed it to sunlight coming through glass. A day or two went by, and the leaves started getting a copper color. THIS is when I got confused, as the coloring along with the black spots seemed to indicate a viral disease... The color was on old leaves and tips of some new leaves. Finally, an old leaf was shed, and some relatively new leaves (not the newest) turned yellow and brown.

I've watered it on occasions when the potting mix got almost dry. The pot is a size too big. It has drainage holes in it, and it's sitting on coconut husk chips and fibers.

The water runs right through the fibers on watering, the husk chips retain some moisture, dries out completely in a little over a whole day (I haven't had the chance to replace the husk chips, reused the harder ones from the mix it came in, added some fresh husk fibers).

Been careful while watering, tried not to get water on the leaves.

DEN-PHAL:

As for the den-phals, they are hanging against a white south wall, on a veranda. Receiving bright indirect sunlight.

Been watering them once every two days, since the mix is old and retains more moisture A few leaves on one are a little wrinkly, this is the one whose flower wilted prematurely. The other one with green spots on the back of the petals has happy leaves. Both plants have happy roots!

Got rid of the spotty leaves. ^This is the current state of the oncidium! I'd isolated it, and moved it to a room that only received moderate filtered sunlight.

^The den-phals in their usual place. Removed the wilting flower on left plant.

^The wrinkly leaves on the white den-phal plant.

The roots. The plants are overcrowded, will repot after they all finish blooming!

Dend/Phal - I would just stick the little basket inside a slightly bigger pot. No need to damage the roots trying to get it out. If the Dens are in bright sunlight, they can probably be watered daily.

Oncidiums - The spotted leaves are not a virus, normal for this type of plant. The copper color could be do to sudden higher light levels, again normal. You should not be cutting off any leaves unless they are dying or diseased. Your plant looks healthy except looks too wet and overpotted. Wait until Spring and repot in fresh mix in a pot slightly larger than the root ball.

You are in zone 11 which is quite hot. Oncidiums like cooler tempertures. I would move the plant out of coconut husks into fine bark. Personally, I do not like coconut for orchids.

Jane

An update on the oncidium. Couldn't get the browning of leaves to stop :( Tried different levels of watering. The roots seemed fine till the very end, the pseudobulbs browned off last. I am guessing either the weather was too hot for the plant, OR, it didn't like the water it was getting (tap water)...

Bought another oncidium, sadly the same issues are appearing. It gets boiled water, can't help the weather... Hoping to be able to save it!

Probably needed repotting...

@shavedmonkey Yeah, old media plus overpotting. But you know, the roots looked fine, even towards the end!! Wouldn't the roots protest to bad media? The plant was overpotted, but it was packed nicely, as in, it was STABLE in it's place... Not an expert on potting, but I read somewhere that an orchid can adjust to overpotting as long as it sits tight (do correct me if I'm wrong!!).

I could only GUESS the problem to be either heat stress or water-hardness... Still not sure what killed it... :(

It is possible that the medium becomes toxic to the plant...