I assumed that the LH2 was one of OPT's basic options, or that OPT was fully compatible with "Interstellar FuelSwitch" or "Cryogenic Engines". Because before 1.3.1 I always had this option for OPT parts. Moreover by installing only OPT I have the option "LiquidFuel, Oxidizer" to double, as if a bug prevented to have LH2 instead of one of the two. But if not, does anyone know how to recover the LH2 for OPT parts? Because half of my ships are stuck without fuel in the space vacuum ^^'