Lay the hydraulic-oil-stained pants on a flat surface. Cover the grease stains with baking soda to absorb the grease. Allow the baking soda to remain on the fabric for 20 minutes.
Brush the baking soda into a garbage can with a clean cloth.
Spray the remaining hydraulic oil stain with an aerosol petroleum-based prewash stain remover. Let the stain remover soak into the fabric for one or two minutes to help loosen the grease.
Wash your pants in the hottest water allowed according to the care label. Add laundry detergent and 1 cup of white vinegar to the wash cycle to remove remaining hydraulic oil stains.
Check your pants for other oil stains before placing them in the dryer. Treat any remaining oil stains by soaking the pants in a bucket filled with 1 gallon of warm water and 1/2 cup of oxygen bleach for 30 minutes. Remove the pants from the soaking solution and launder them again.