Learning mathematics from MIT OCW? Is it enough? I sorely regret my undergraduate career choice of pursuing biology. As such I've embarked on my quest to study mathematics with the use of MIT OCW. Is the MIT OCW resource enough to get a firm grasp of what is actually happening? Let me rephrase that. Can I learn enough to go beyond just solving problem and actually understanding the mathematics of it?
 A: I've learned quite a bit of what I know today from various online venues, i.e. in absence of a "formal class." The issue I think is not whether it is "enough," but whether you will have the dedication to actually take it upon yourself to do what you need to do to fully understand what is going on. In a formal class, you are more or less forced to do this since your assignments are graded and the instructor typically formulates them with the goal of understanding in mind. Without an instructor, you must take on this role, which can be hard since you don't necessarily know which exercises are more important than others in terms of understanding. Luckily, OCW often has pre-formatted problem sheets, made by the instructors, so you could just use these. In short, I think it's an issue of discipline rather than availability of information. I was able to do it in many cases, but basically every time I had a thought like "I don't need to do this problem, it's trivial/irrelevant/not related to what I want to study," I had to force myself to do the problem, as you should as well.
A: I'm going to take a different route around this question and make the bold statement (pun intended): I don't know, what do you want to do with math?
I would argue very strongly that if you're idea is to go into mathematical research, or work at a federally-funded lab, or eventually get a PhD... then, no, I don't think online courses are sufficient, for the main reason that a university education puts you into contact with professors that you have a right to access, in contact with research resources, and ultimately in contact with people who can get you into graduate school.
However, if you're goal is to go into mathematical biology, or perhaps enter some math-esque field, I would agree with M10687's answer, and perhaps go a little bit further. Online courses, especially the ones offered on OCW, are very-well made. Like all universities there are some good professors and some bad professors, but I think the quality on OCW is better-than-average.
That being said, I would argue that you should augment whatever online resources you use with a physical textbook. I'm an engineer, not a mathematician, so I always like to map abstract topics to some real-world analogy, and textbooks are generally a great guiding tool. If you're looking to learn applicational mathematics (multivariable calculus, differential equations, probability, etc.), textbooks are a good bank of problems. If you're looking to learn abstract mathematics (algebra, number theory, set theory), textbooks provide a good way to read the information in a rigorous way, which lectures don't always do well.
If you are still attending your university, I would also immediately get into contact with mathematics professors and ask for textbook recommendations, as well as find out their office hours. Most professors (in my experience) don't get very many people attending office hours, so I'm sure it'll be a breath of fresh air if you were to ask them questions about whatever class they're teaching.
Finally, I think an important part of learning anything is being able to have a metric to gauge your understanding. OCW has supplementary material a la homework sets and exams, and it would behoove you to take these as seriously as possible. There have been times where I didn't have the motivation to pretend that topic XYZ was a real class that really affected my GPA, and my learning suffered because of it. Take notes during lectures, do the problem sets without looking at the answers, and punish/reward yourself for exam grades how you normally would.
tl;dr: if you're trying to learn math for a job in mathematical biology, OCW is a great resource if you augment it with professors, textbooks, and motivation. On the other hand, I argue that this isn't sufficient for a PhD position or mathematical research. To pursue those routes, consider going back to university.
