Adopting a variational approach, this self-contained textbook presents the basic principles of optimal control governed by ODEs. Inside, readers will encounter the fundamental issues of existence of optimal solutions, optimality conditions, and numerical approximation from a variational perspective, academic and real-world examples testing the book's variational approach, and a unified treatment that requires no prior expertise in variational problems or numerical approximation. A Variational Approach to Optimal Control of ODEs is for applied analysts, applied mathematicians, and control engineers. It can be used in courses on Advanced Numerical Analysis, Special Topics on Numerical Analysis, Topics on Data Science, Topics on Numerical Optimization, and Topics on Approximation Theory.