Butter is, generally 80% fat, 20% water. So, if your recipe calls for 1c butter, you’d want to replace it with just over 3/4 c. oil just under 1/4 c. water. Or, replace 100g butter with 80g oil + 20g water.
I would definintely try this privately, though, before creating these for an occasion. Brownies are so butter-rich, they could be distinctly different with oil—maybe in a good way, but maybe not!!! For example, I find truffle centers made with oil kind of disgusting, so the replacing dairy fats can work in some things, but not others, and you’ll definitely want to confirm for yourself!! You can also try nut oils which will add a different kind of flavor.
You could try a tiny partial batch in a mini loaf pan or something.
Let us know how it comes out!