Oxaloacetate + Acetyl CoA ---> Citrate + CoASH. This reaction is:

In citric acid cycle, the initial reaction between acetyl-CoA and oxaloacetate to form citrate is catalyzed by citrate synthase, which forms a carbon-carbon bond between the methyl carbon of acetyl-CoA and the carbonyl carbon of oxaloacetate. The thioester bond of the resultant citryl-CoA is hydrolyzed, releasing citrate and CoASH--an exothermic reaction. In the subsequent reactions, two molecules of CO2 are released and oxaloacetate is regenerated; hence oxaloacetate can be considered as playing a catalytic role. Ref: Bender D.A., Mayes P.A. (2011). Chapter 17. The Citric Acid Cycle: The Catabolism of Acetyl-CoA. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.