If you take magnesium oxide to help with leg cramps, does it make your muscles weak?

Should not. Mgo is used for relief of indigestion, heartburn as an antacid; short term laxative; and supplement. Side effects may be nausea, abdominal cramping, and long term use may cause bowel obstruction. It should not result in muscle weakness.
Yes, if take too muc. Magnesium oxide 420mg once/twice daily should not make muscles weak but if you take large amounts and have kidney failure, your magnesium level may get very high and this may be dangerous / cause weakness. It is possible that 420mg once daily could cause symptoms of weakness, but I have not seen this.