Olive oil is better than vegtable oils, but the best is flaxseed oil. Also you can't properly season cast ironware with only one application. After you have the pot clean, wipe a light coat of flaxseed oil inside and out and as mentioned place in a hot oven or on a stove burner over low heat. When it quits smoking, allow the pan to cool and repeat the application 8 or 10 times. In some cases more coats are needed. The ultimate goal is to build a smooth finish of baked on oil so the metal is not only protected, but is smooth as well.
For example we have a large skillet that was my grandmothers, and seasoned in the manner above by my granddad. it has never had to be reseasoned and scrambled eggs for example slide right out after cooking, requiring only a wipe with a damp rag to clean the skillet after use.
Speaking of cleaning cast iron, never put them in the dishwasher or a sink full of dishwater. Properly seasoned, all they require is wipe them out with a slightly damp rag or wipe them with a dry towel then a slightly oily (with flaxseed oil) paper towel then buff them with a dry towel.