In recipes that call for butter (especially melted butter) can i substitute oil instead?

What kind of recipes?? Some you can and some you can't. Give us the names of the recipes so we can help you better!!

Chicken and fish recipes mostly. They often call for both oil and butter, so i'd need to use just oil for all of it.

If you are using just oil for cooking meats, poultry, fish, etc. - I don't think it will alter the flavor and ruin your dish!

Many recipes that call for butter and oil - I use just oil - no problems.
:p

I always substitute canola and olive oils for butter when frying foods or baking, or marinating. In fact those are the only oils I use. I don't think it would work for cookies, and cakes. Probably okay with breads.