Why publish your paper Open Access? Here are a few reasons:
According to WUSTL Intellectual Property Policy, students own copyright on their work. [see sect.3.b.2 and 2.b.3] There is no requirement to register your copyright nor to mark Copyright © and the year on the document. That is entirely up to you.
Especially for open access sharing, you may wish to consider choosing an explicit license which specifies how you want your work to be used, whether you want attribution, etc. Creative Commons licenses are one way to make your wishes clear.
It is common that research might include pictures, graphs, art examples, or other items which have been published elsewhere. Since you may wish to have your work available in Open Scholarship, you must ensure that you are including materials consistent with the rights of the owners of that work.
Some of your options: