Why are enzymes called a lock and key situation

Because enzymes connect to their substrate through a lock-and-key system, where the shape of the substrate and the enzyme has to fit like puzzle pieces. Then only the enzymatic reaction can occur.

ability to bond woth oxygen

an active site is a region that is present on an enzyme where the reactants or the substrate molecules we can say bind and experience a chemical reaction. the substrate is basically a reactant whose concentration is changing and that is converting into a product after binding at the active site of an enzyme