will this lens fit my Olympus om de m5 camera? using in underwater housing for same camera, do I need a dome type port?
Yes, the m43 lensmount has not been modified since it was introduced and this lens will fit any m43 Olympus or Panasonic body including the OM-D EM-5. I own the lens and find it quite versatile with acceptable sharpness and acceptable field curvature which can be corrected in post processing. If you are planning to shoot underwater, field curvature probably won't bother you too much,
However, I have no idea whether the lens + body fits any underwater body with or without a dome port. I would think you'd need to make inquiry to Olympus or to whatever underwater housing manufacturer you are contemplating.