Refilling your OMAS rollerball pen is a snap if you have the old refill. Just measure the length of the old refill. You will either find it measures 3 7/8" or 4-5/8".The shorter refill (3 7/8") can be replaced with the OMAS brand of rollerball refill. We suggest this option as certain OMAS rollerball pens do not fit the Schmidt (P8126) as well.If you have the longer version of refill (4-5/8"), then the Schmidt 8216 option will work in your OMAS rollerball pen. Only the early OMAS rollerball pens utilized this longer option.If you are overwhelmed or do not have the old refill and would like a 'hand', give us a call at 800.766.7367.