I've graduated and I'm looking to apply to a Ph.D. program in computer science. However, during my undergraduate years I did not take math courses other than the ones required for the CS major (i.e., discrete maths and algorithms). Thus I haven't taken a college course in calculus/statistics/linear algebra/probability, which I am aware is essential training for CS Ph.D. students. I also haven't taken an operating systems course since it wasn't required for the major and I opted for a different set of CS courses.
The reason for this was that I did not consider applying to graduate programs at the time, and chose to take courses in other unrelated departments to fulfill requirements for my second area of study. However, I am currently taking an online MOOC in calculus and linear algebra to increase my technical knowledge.
Although I have good grades in my CS courses and research experience, I'm wondering if this would be considered a red flag that I should address in my statement of purpose. If so, what would be a good way to do it?
Alternatively, would it make sense to apply for a Masters program to get more coursework under my belt before applying to a Ph.D. program?