What is the name of the enzyme that catalyzes the reaction that joins a molecule of acetyl-CoA and oxaloacetate?

Citrate synthase is the enzyme you are looking for.
Because it regulates the first step of the Krebs cycle, this enzyme is a key regulator
of rate of respiration in the mitochondria of cells. The reaction it modulates is a
condensation reaction. It is believed that the acetyl Co-A attaches to the citrate
synthase molecule first. This attachment changes the enzyme and increases its affinity
for oxaloacetate by a factor of about 20. Once the oxaloacetate binds to the enzyme, the
citrate synthase molecule changes shape and becomes nearly round, bringing the acetyl
Co-A and the oxaloacetate into the correct position to bond, producing a new six-carbon
molecule of citrate. The citrate molecule is then released into the Krebs cycle. Citrate
synthase activity is inhibited by high levels of ATP, and also by the presence of its
own product, citrate.