Heat the oil in a 3-quart saucepan over medium heat. Sprinkle oil with salt to taste. Put a few popcorn kernels in the oil and cover pan.
When kernels pop, add remaining kernels in an even layer. Cover pan and remove from heat for 30 seconds. Return pan to heat and wait for popcorn to begin popping, shaking pan gently and keeping the lid slightly ajar to allow steam to escape. Once the popping slows to several seconds between pops, remove pan from heat and pour popped popcorn into a large bowl.
Place butter in now-empty saucepan and melt. Pour over popcorn. Add more salt to taste, if necessary.