I've been accepted into an MS program, having never taken an I/O course. You'll want to take as much stats and psychometric courses as you can. Get involved in research as a research assistant. The best way to do that is to email professors, hopefully ones who know you already, and ask them if they have any openings to assist in research for credit. I went to a very large university (Ohio State) and so few students took advantage of this opportunity. I actually recruited three others for the lab I worked at. The research doesn't need to be I/O specific; research skills are transferable. However, I/O research would look great as would I/O courses, so take those if you can fit them in your next two years. Other courses such as Organizational Development or the like that are usually offered in business schools would be beneficial too.
Find an internship in HR. If you can gain both research and real world experience, you'll be above and beyond your peers. The professors I talked to said this was a rare combination and considered these applicants more favorably.
Consider a MS or MA program. As long as its not terminal (i.e. completely practitioner focused so does not qualify you to apply for PhD) you'll have the option to apply and move on to a PhD. This would be a good option if you graduate in two years without any publications or little research experience, which is what a PhD program would require. I assisted many studies but only conducted one study myself, which was for a course and never published.
In summary, take I/O courses, stats and psychometric courses, talk to I/O professors and get involved in research and find relevant internships.