Oncidiums can be constantly growing, especially mature ones. As others have said, they often start new growth while in bloom.
I water my Oncidiums normally all the time. They don't require rest periods. That said, remember that watering frequency varies with different weather. - cooler weather - less frequent watering ; tho if heat is used a lot, may need as much or more watering as other times of year. Using a skewer helps determine when to water.