oncidium leaves yellow

It's almost impossible to diagnose from a photo, especially when no description of the current cultural conditions or specifics of the feeding and watering regimen are given, but I'll throw out some possibilities: excessive sunlight, insufficient nitrogen, insufficient magnesium.

 

Due to the a.m. possibilities
I think you should check the roots.

 

"above mention".

 

it would really help if you told us the details:

• what is the formula of the spray fertilizer and to what dilution rate do you mix it?
• Do you apply the “solid” undiluted???

 

We’re not communicating well, and you are likely overfeeding by a huge amount! What is the formula of the pink stuff? Or is that the 16% ?

No powdered fertilizer should be applied dry, then watered in. Think about it: you’re applying a 16% N fertilizer (160,000 ppm N) at close to full strength and the velamen grabs and holds onto that first solution. Your subsequent watering does not adequately strip it down to a more appropriate concentration, as the velamen is designed to hold onto nutrient ions and prevent that.

if you feed every two weeks, shoot for more like 250 ppm N, already in solution. Divide 20 by the %N to get the teaspoons/gallon to mix, or divide 23 by the %N for ml per liter.

For a 16% fertilizer, that would be 20/16=1.25 teaspoons gallon or 23/16=1.4375 ml/L, which would be a bit under 1/3 teaspoon/L.

 

I tried to find it online, and it turns out to be a chitosan/seaweed extract, not a fertilizer, per se, so probably OK at that rate.

 

As far as reading the labels in Thai... Google Translate on your phone can help a lot. (I use it for translating cooking instructions on some of the products that I get from my Japanese grocery, that don't have translated directions, it can from from images.)