Please note: Many internet sources which discuss ancient American scripts do not clearly distinguish between 'Olmec' and 'Epi-Olmec'. Epi-Olmec (which means 'after-Olmec') refers to a script which dates from the second century AD. It's possible that the natives who wrote the Epi-Olmec inscriptions were direct descendants of the Olmec, but not everyone considers this proven.
Recently, anthropologists from Florida State University, Pohl, Pope, and Nagy, described symbols which they found at an Olmec site near La Venta, Tabasco, on stone and ceramic. According to them, these symbols, which date from the seventh century BC, represent a written language. However, other experts on ancient Mesoamerican culture (Coe, Marcus, Stuart) remain skeptical.