Furniture oil is not the same thing as a sealer. However, it does greatly improve the ability of wood to resist moisture. Because all types of oil will naturally repel water, it will act as a sealer to some extent. In the days before wood sealers and clear coats were invented, oil impregnation served as a useful technique for preserving the life of a wooden object.
At the same time, you cannot expect a furniture oil to function as well as a sealer. Both of these products will penetrate the surface of the wood and provide a waterproof top layer. However, most commercial wood sealers are plastic-based and made of synthetic polymers. This is both a good thing and a bad thing, depending on your perspective. On the good end, plastic will last a lot longer than oil, which has to be re-applied once or twice a year. On the bad side, plastic is not environmentally friendly at all. Furniture oils tend to be made from natural substances, but this also increases their price somewhat.