There is a difference in that ollas as you know are sealed containers with a hole in the top to pour the water into. Clay pots will do the same thing but it takes some preparation as you need to create your own olla. Here is a website that teaches you how to make one step by step. Making them yourself is less expensive but regardless of the method you use you will need to glaze or paint the portion that extends above ground to keep the moisture from evapoating via the pot pores.

Here is a link that might be useful: Making an olla from clay pots