what would be the impact on the supply curve of oil if a major oil-producing country banned all exports to the united states?

If a country that is a major oil producer decides to ban all oil exports to the United States from some reason, the supply curve would go down most probably.

The United States are the biggest economy in the world, and they are also one of the biggest importers of oil in the world. The supplying of oil comes from most of the major producers of oil in the world. If a major oil producer bans the exporting to the United States than the supply curve will be going down almost certanly.

The variable here is only by how much the supply would go down. That would depend on how much oil the country was exporting to the United States. The only way in which the supply curve would not go down is the country manages to find another country that would import the same amount of oil as the United States did, and that is pretty hard to achieve in practice.