Check fees, if any: how will you pay article/author publishing/processing charges (APCs): grant funds, other, request waiver
Check license: this varies; some OA journals do not offer a Creative Commons license
Investigate other info about prospective journal: where indexed, quality indicators, etc.
"Although charging publication fees is the best-known business model for peer-reviewed OA journals, it's not the most common. Only about 30% of peer-reviewed OA journals overall charge publication fees. When OA journals do charge fees, the fees are usually paid by funders (59%) or by universities (24%). Only 12% of the time are they paid by authors out of pocket." [From How to make your own work open access CC_BY 3.0]
Washington University does not have a fund to assist with open access article charges. It's possible that some departments may have funds for this, so you may want to ask. The libraries have a few memberships, etc., which maydiscount charges with some journals.