The planet you speak of is dry but for some oasis' and it has life? Well, let's make sure that it has no water on the surface. But this also leads to the fact that it can't have developed any life that is water-based on its own. No, let's skin the cat a different way and have the planet turn desert only after some point in time.
The pre-historic Arrakis
Before people were there, the planet had a mixed surface. It was quite flat, but there also had been quite more water, creating an expansive band of rainforest around the equator. In it, people developed over the generations. But then the He-under-the-sand arrived in the north desert, possibly by a comet strike. Nobody noticed at first, but over eons, Father Eternity did change the climate slowly as his children spread over the pole.
Shai'Hulud consumes water from the surface: instead of excreting water, they excrete a hard substance called Melange. But as he grows and gets older, at some day he dies in the deep strata, leaving the water he had gathered there. As his body decomposes, the water is reintroduced to the deeper layers of the earth, creating water veins.
Arrakis as we know it
Over ten thousands of years, the water got stolen to deep earth, and the planet underwent desertification. People developed culture and started to cope with the dwindling ecosystem as The Makers prospered. The last remains of the old rainforests are oasis' that are protected by sitting on bedrock and not on the sand like the rest. Even those are shrinking as their water supply is drained underground and into the atmosphere slowly.
It will take the invention of deep pumping to secure those oasis' by replenishing their water from the deep strata, but for the time they exist.