FO Otter time
Yes, it can be tricky....to log FO time on a US license it has to be an operation that requires two pilots (ie; 135 IFR with 10 or more pax; 121 ops etc) or an aircraft that requires two pilots (in the US, this is not an Otter). The company requiring it doesn't count, only government regs. If the country you are operating in requires it, I think that might do. Call OK City and ask...
If you can't log FO time, then you can always log PIC while sole manipulator of the controls (this doesn't mean you are acting as PIC, just that you are allowed to log it, as two pilots cannot fly the plane at the same time). Many Capt.s won't like this as it implies (correctly) that they cannot log the time that you fly.