Manuel Antonio de Oca y Alemán, who commanded San Luis de las Amarillas Presidio (San Sabá) in its final stages, the son of Josef Antonio de Oca and Manuela de Alemán y Oribe, was born in the first half of the eighteenth century at Logroño, Spain. Both his parents and his grandparents were "old Christians" of racial purity, as the term was applied in the Spain of that day. Educated at the University of Hirache, Oca graduated as a bachelor of laws, but circumstances led him into a military career. After serving four years as a cadet in the Queen's Infantry Regiment, he was made lieutenant colonel of the Logroño militia. He went to New Spain in 1758 with a two-year crown appointment as alcalde mayor of Sayula province. His duties, which he is said to have discharged with notable efficiency and integrity, included collection of royal tributes. From Sayula, Oca was summoned by the viceroy, Marqués de Cruillas, to service in the Seven Years' War as a subaltern. In recognition of his military service, he was made commandant of the province and presidio of Nayarit, where he served four years before being sent to Texas. Ordered to relieve Felipe de Rábago y Terán as San Sabá commandant, Oca reached San Fernando de Austria in northern Coahuila, where he encountered Rábago and assumed command on April 1, 1769.
Rábago, in failing health, had abandoned his post to relocate his garrison in the Valle de San José (the upper Nueces River, also referred to as El Cañón), the site of two struggling Apache missions founded in 1762. Oca had scarcely arrived at El Cañón when the log stockade was attacked by "the Indians of the north." After a three-hour battle on May 22, 1769, the natives were repulsed without the loss of a soldier. Later the same month Oca agreed to station thirty of his soldiers at San Fernando to protect that area while Manuel Rodríguez drained Coahuila's military strength for an Indian campaign to the west. From a skimpy reference in correspondence between the Coahuila governor and the viceroy, it appears that Oca took whatever was left of his company back to San Sabá but left the post for good early in 1770. No mention of the trip or other details of his command of the San Sabá garrison appear in the account of his services dated 1773. Nor is there mention of the charges, of unspecified nature, brought against him by five soldiers of the garrison, still at El Cañón, in 1771.
Ultimately, the San Sabá company was divided between San Antonio de Béxar and Coahuila, where part of it was used to establish a new presidio, called San Vicente, in the new defense line established in 1773. Oca, however, apparently had no part in it. Nothing more is heard of him on the northern frontier.