Water migrates back and forth between the oil and cellulose depending on the temperature (both can hold different levels of moisture depending on the temperature). So there is never an equilibrium. High water in the oil can in the worst case cause dielectric failure of the transformer but more generally causes accelerated aging of the transformer cellulose. If the temperature of the transformer oil is known when the oil sample was taken, there are saturation curves that can be used to determine the percent moisture in cellulose and there are also industry guidelines for what is high moisture in cellulose. Thus it is important to watch the moisture in oil and perform processing / dryout to reduce the level when required.
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