Can I first expand osteoblasts and then perform my experiments?

Yes, normal human osteoblasts (HOB) can be expanded in vitro to a limited extent before being used for experiments.
Like other non‑transformed primary cell types, osteoblasts have a finite proliferative capacity. In most laboratories, HOBs are therefore used at low passage numbers, typically up to passage 4 (P4), to ensure reliable and physiologically relevant results.