If you mean the 25mm Olympus lens, which has a 46mm filter thread, then look no further than the excellent polar pro filters. I purchased the 6 filter set & use the ND8 & ND16 very frequently.
I recently purchased the Laowa 7.5mm lens and it came with 3 ND filters as part of the package deal from Adorama. They’re a CPL, 2 ND, and the always useful UV filter. Of course they’re 46mm thread so you have to get a 37mm to 46mm adapter but they are quality multi coated glass filters and I have had no trouble with them.