What are some good online resources that will help me better understand BigO notation, running time & invariants?
I'm looking for lectures, interactive examples if possible.
Since @Pratik's answer covers good resources for algorithms, I just wanted to mention that since you asked about big-O notation, Peter Braß has written a book on the topic, which you can find here.
Update: In response to the comments, I recommend this book for beginning students who are having trouble wrapping their head around big-O. (Yes, every algorithms book ever covers it to some extent, but my experience is that one of the most common questions on math.SE is still basically "I am having trouble with big-O", so there must be a place for a more expansive treatment.)