I finished the OMSCS program while working full time. I think most can do it unless you regularly work significant overtime or otherwise have a significant lack of free time. Those with families, kids, etc. seem to do best taking one class per semester. Those with fewer such responsibilities can take two per semester.

Also: Even at two classes per semester it takes two years. One class per semester, a bit over three years. It does take some determination to see it through to the end.