You can contact a seller advertising an item with the number provided besides the advert or by sending them a direct message or chat. With the new OLX, you can now share recent images and get Google Maps directions in chat without giving away any of your private information. Better yet, you can also share voice notes too.
Here's a quick safety tip:
Once you've found the perfect item for you on OLX, make sure to meet the seller in a public location. Take your time to look over the item thoroughly before you hand over any money.