Some Oncidiums are known for having lots of spots. You're using relatively pure water, however, so that eliminates the most prominent cause of leaf spots. And they usually aren't so necrotic, with large depressed areas.
I suggest not using fungicides unless you know what you're treating. It kills off beneficial organisms that inhibit pathogens. And many fungi are resistant to various fungicides. You need to know what is the fungus and whether it's susceptible to the fungicide used.
I rarely say this, but I suggest you go to the diseases section of the St Augustine Orchid Society Web page and look at the virus photos. It's impossible to tell whether a plant has virus infection without a test, but in your conditions I think it's a possibility.
You could also take a piece of leaf to a local university or your Agricultural Extension office. They may be able to look at it under a microscope and identify the problem.
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