Are brassica oleracea cultivars the same species?
Cultivars are, per definition, the same species of plants. Animal breeds are the same species too, think of the bewildering phenotypical variety of dogs for example; they are still the same species, i.e., Canis lupus.

To your question - Yes, cabbage, broccoli, cauliflower, and Brussels sprouts are the same species.

Hybrids of these plants are called cultivars; broccoli and cauliflower yields broccoflower for example. However, I wasn't able to find evidence of a hybrid between cabbage and cauliflower - a cabbaflower?