Absolutely not. Dextrose is a simple sugar. Table sugar (sucrose) is a disaccharide of glucose + fructose. Heating it won't effect the sugar molecule. It takes very high heat (300 degrees celcius+) to change the molecular structure of sugar.
Mixing dextrose into your warm oats would be the equivalent of mixing sugar into a hot cup of coffee. Not an issue at all
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What is your reason for wanting to mix dextrose into your oatmeal?
There are much better options in my opinion...things like blueberries or a banana