Ouwens was the child of Pieter Anthonis Ouwens, accountant in Amsterdam, and Caroline Reiniera Nagels. He studied at the Royal Military Academy in Breda from 1867. In 1871, he became lieutenant at the Infantry in the Dutch East Indies. In 1883 he was promoted to captain. In January 1879 he married Johanna Vosmaer at Banda Aceh. They divorced a month after the birth of their only child in 1882 in Salatiga. Ouwens then married Jeanne Dikkers in December 1883 in Purworejo. He married again in 1902 with Anna Josephina Soesman.