Bifunctional sesterterpene synthase; part of the gene cluster that mediates the biosynthesis of the sesterterpenes ophiobolins, fungal phytotoxins with potential anti-cancer activities (PubMed:27116000). The first step of the pathway is performed by the sesterterpene synthase oblA that possesses both prenyl transferase and terpene cyclase activity, converting isopentenyl diphosphate and dimethylallyl diphosphate into geranylfarnesyl diphosphate (GFPP) and further converting GFPP into ophiobolin F, respectively (By similarity). Other sesterterpenoids (C(25) terpenoids) are found as minor products of oblA (By similarity). The cytochrome P450 monooxygenase oblB then catalyzes a four-step oxidative transformation of ophiobolin F to yield ophiobolin C (PubMed:27116000). The FAD-dependent oxidoreductase oblC might be involved in a later oxidation step that produces ophiobolin A (Probable). (2E,6E)-farnesyl diphosphate + isopentenyl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + diphosphate (2E,6E,10E)-geranylgeranyl diphosphate + isopentenyl diphosphate = (2E,6E,10E,14E)-geranylfarnesyl diphosphate + diphosphate (2E,6E,10E,14E)-geranylfarnesyl diphosphate + H2O = diphosphate + ophiobolin F Binds 4 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. The conserved DDXXD motifs as well as the NSE/DTE motif are important for the catalytic activity, presumably through binding to Mg(2+). In the N-terminal section; belongs to the terpene synthase family. In the C-terminal section; belongs to the FPP/GGPP synthase family.