Go to this website: http://www.spanamwar.com/http://www.spanamwar.com/in search box type "Louisiana" and read about some units. There is the "1st U.S. Infantry" that "mustered in" between May 25th and June 25, 1898 and "mustered out" June 22. 1899.
Found him and family in 1920 Louisiana, Lafayette Parrish, Police Jury Ward 8. Surname is transcribed as "Decone" and "Odillon Decone" is listed as age 46 b. Louisiana/parents Louisana occ: farmer, wife Eva is 39 and 6 children in household.
Was he married a second time to an Olivia? In 1930 Louisiana, Lafayette Parrish, P.J.#6 I find a "Adillion" (written it is Odillion) Decou age 59, frst married at age 23, b. LA/father- LA/mother-France occ: truck farmer and Spanish Americian War Veteran with a wife listed as Oliver ahd 58, first married at age 17 b. LA/parents LA. The LA Death Index has a Olivia D. Decou d. 1847 listed after Odillon Decou with same death date that you have, both in Lafayette Parrish.