Final answer:
The conversion of pyruvate to oxaloacetate occurs in the mitochondria during the Krebs cycle.
Step-by-step explanation:
In the process of cellular respiration, the conversion of pyruvate to oxaloacetate occurs in the mitochondria. After glycolysis, the pyruvate molecules are transported into the mitochondria where they enter the Krebs cycle, also known as the citric acid cycle, which takes place in the matrix of the mitochondria. This is the second major stage of cellular respiration.