Do you have a photo of the refill currently inside it? Or do you have the model number?
I have two OHTO pens that use OHTO refills. One is a FLAT-C that uses a PG-105NP (which is like a Parker G2 style). The other is called a “Needlepoint”, model NBP-507SH, but it uses a very slender refill, like the straight-tube type: 897-NP 5K.