I need melted butter

Butter is at most 82% oil and the rest is water. So, you're going to need to add more butter for the same amount of oil. Besides, you need to adjust the liquid content accordingly since you are going to have more water than you planned.

If you are going to use the same amount of oil, then it will not make any difference other than the taste. Just make sure they are of same state (i.e. melted butter instead of vegetable oil or vice versa)

If the texture changed immediately, then that sounds like seizing to me. Chocolate shouldn't seize if you add fat (oil) to it, particularly if you add it after the chocolate is fully melted. Only water will cause that.

Olive oil varies significantly in purity and quality, so cheap olive oil may actually be worse for this application than a purer, cheaper cooking oil (such as peanut or corn oil). There may have been impurities or even trace amounts of water in the bottle. Or it could be that you just tried to melt it too fast, or you accidentally got some water in there from some other place (condensation is a common source).

If you melted it directly in a pan, as in your comment to Kyra, and continued to apply heat after adding the oil, then it's also possible that it didn't really seize at all and that you just tried to melt it too fast.

If it did seize, then there is a cure - perhaps counter-intuitively, adding more oil could help you. Using roughly a tablespoon for each ounce of chocolate will help smooth out any lumps. Add and whisk it very gradually. Of course, this will only work if the oil itself is reasonably pure.

Note: In this case, I would guess that the use of low-quality chocolate isn't your problem if you successfully melted it (albeit to a thick consistency) before adding any oil. Significant impurities in the chocolate itself would lead to seizing during the melting process.