You can use olive oil to darken leather, as the consultants and engineers of Delhi’s National Institute of Industrial Research point out in their manual on leather tanning. However, olive oil is primarily manufactured as a food product. All oil will darken leather to some extent. Some people approve of using olive oil as a “non-toxic” alternative to chemical-based products.
Pour a small amount of olive oil into a saucer. Avoid staining, or uneven application, by using just a little oil at a time. Dip a clean rag or brush into the olive oil, taking up a little. Work the oil into the surface of the first boot.
Dip the rag or brush into the oil again when necessary and proceed in like manner over the whole of the first boot, taking particular care with any patterned areas. Set the boot aside to dry. Treat the second boot in the same way.
Return to the boots every now and then to check them. Touch them to see if they are still sticky. When the oil has soaked in and the boots are just tacky, you can buff them up.
Rub the leather with a soft polishing cloth, round and round and back and forth across the surface. Aim to remove any excess oil applied in error. Buff every part of the boots as required to create a shiny and even finish. The boots will now be darker and glossy.
Take the advice of website Sonssa and test the olive oil on a small area of the boot first.