Marriage to a U.S. citizen is a common path to permanent resident, i.e., a green card, which, in turn, may lead to naturalization and citizenship. An order of deportation is an obstacle to a green card. And by "illegal" what do you mean? Did this person enter the U.S. with a visa and a passport and were he or she inspected by an immigration officer? Did he or she overstay a visa? Has this person ever had any problems with the law? Is this person currently living in the U.S. or abroad? These are some of the many questions that I would want to know the answer to in order to assess this case.