Hydroxamic acids are usually prepared from either esters or acid chlorides by a reaction with hydroxylamine salts. For the synthesis of benzohydroxamic acid ( or , where Ph is phenyl group), the overall equation is:
Hydroxamic acids can also be synthesized from aldehydes and N-sulfonylhydroxylamine via the Angeli-Rimini reaction.  Alternatively, molybdenum oxide diperoxide oxidizes trimethylsilated amides to hydroxamic acids, although yields are only about 50%.  In a variation on the Nef reaction, primary nitro compounds kept in an acidic solution (to minimize the nitronate tautomer) hydrolyze to a hydroxamic acid.  
A well-known reaction of hydroxamic acid esters is the Lossen rearrangement.