As it turns out, ollalieberries are a West Coast thing. Developed in Oregon in 1935, they are a cross between a loganberry and a youngberry. Both of these parent berries are crosses of blackberries with raspberries and dewberries, so it's kind of like the blackberry is the "grandparent" of ollalieberries. The berry didn't grow so well in Oregon in spite of being developed there, but it has been prolific in California.
These berries have a very short seasonal window; they're usually available for two or three weeks at the beginning of June, depending on weather conditions - generally, they're available between May and July. They look like blackberries in that they are clusters of black globes filled with juice and seeds, but they are longer and slimmer than blackberries and just as sweet, but a little tangier.
They can be used in any recipe calling for blackberries, from pies to cobblers to jams.
(Image: Riley's Farm, a working farm in Southern California where you can pick your own fruits and vegetables.)