Your best bet is to call the front desk, they'll be able to tell you what the hours of the Oasis Cafe are for the time you'll be visiting. My recollection is that the Cafe has been open for lunch every day, even weekdays, while we've been there during the high season. We're not drinkers, so I don't know if the bar is run separately, but if the Cafe is open the bar has to be too.

This link from the DS website I believe is outdated, which is why I think you should call the front desk to be sure.
http://www.desertsunresort.com/amenities/restaurant