Garlic oil is a byproduct of roasting garlic and is commonly used in restaurants to create a more flavorful oil.
I usually buy the peeled garlic for this when I make it at home due to the large amount of garlic I used.
Fill a bread or similar sized pan halfway with peeled garlic then add olive oil until it covers it by at least 1/2 inch. The garlic that is uncovered will cook more quickly and may brown too much if left alone.
Put the pan in the oven with foil loosely on top at 350 for at least 45 minutes. You are waiting until the garlic is soft and mashable. It will be a long while before the garlic is burnt. I usually check every 10-15 minutes from here on out, though you can lightly roast the garlic until it has not really changed color, or you can wait until it is a golden brown.
Cool the pan once it is done so you can put the strained garlic oil into a container that won’t melt.
Either keep the garlic as is, or purée it and find the many useful ways such as marinating chicken or making garlic bread.