Registration is required to submit or update papers to arXiv.
Note that besides personal data, the registration form asks the user to specify “Groups that you'd like to submit to:“, among the following options: cs, math, physics, q-bio, q-fin, stat.
In order to submit a paper to the cs.SY category with a cross-link to math.OC, or a paper to the math.OC category with a cross-link to cs.SY, you have to include both “cs” and “math” under “Groups that you'd like to submit to” in your registration. If you have registered with only one group, you can easily add another group by clicking the option “Change User Information” through “My Account” after logging in. To add a cross list to a past submission to arXiv, go to your user page on arXiv, where you may see a list of your past submissions. You may then add a cross list for each of your submissions by clicking the option of “Cross”.
More detailed instructions on submitting, replacing and withdrawing an article can be found at the help site of arXiv, under “Submission and Revision” http://arxiv.org/help/