A new theory of programming is proposed. The theory consists of OE (Operation Expression), SP (Semantic Predicate) and A (Axiom), abbreviated as OESPA. OE is for programming: its syntax is given by BNF formulas and its semantics is defined by axioms on these formulas. Similar to predicates in logic, SP is for describing properties of OE (i.e. programs) and for program property analysis. But SP is different from predicates, it directly relates the final values of variables upon termination of a given OE with initial values of these variables before the same OE. As such, it is feasible to prove or disprove whether a given SP is a property of a given OE by computation based on A (Axioms). SP calculus is proposed for program specification and specification analysis, that is missing in software engineering.