When oxaloacetic acid (OAA) and Acetyl-CoA combine, they form citric acid, also known as citrate. This reaction is the first step of the citric acid cycle (also known as the Krebs cycle).

Here's a simplified breakdown:

* Oxaloacetic acid (OAA) is a four-carbon molecule.

* Acetyl-CoA is a two-carbon molecule carrying an acetyl group.

* Citric acid is a six-carbon molecule formed by the combination of OAA and the acetyl group from Acetyl-CoA.

The enzyme citrate synthase catalyzes this reaction, which is an irreversible step in the citric acid cycle.