This disclosure relates mRNA therapy for the treatment of ornithine transcarbamylase deficiency (OTCD). mRNAs for use in the invention, when administered in vivo, encode human ornithine transcarbamylase (OTC), isoforms thereof, functional fragments thereof, and fusion proteins comprising OTC. mRNAs of the invention are preferably encapsulated in lipid nanoparticles (LNPs) to effect efficient delivery to cells and/or tissues in subjects, when administered thereto. mRNA therapies of the invention increase and/or restore deficient levels of OTC expression and/or activity in subjects. mRNA therapies of the invention further decrease levels of toxic ammonia associated with deficient OTC activity in subjects.