Salted butter can be readily substituted for oleo at a simple one-to-one ratio. "Oleo" is a less-commonly used word for margarine, a form of imitation butter.
Depending on the application, there are other possible substitutions. One cup of lard combined with 1/2 teaspoon of salt can readily substitute for 1 cup of oleo. Additionally, 7/8 cup of either vegetable oil or lard can be combined with 1/2 teaspoon of salt to substitute for 1 cup of oleo; the reduction in volume is necessary because butter and oleo have higher water contents. For sautéing, olive oil is a suitable substitute, although the flavor contribution is somewhat different.