If it is left uncovered, it will get very dry. Do not store garlic in oil at room temperature. Garlic-in-oil mixtures stored at room temperature provide perfect conditions for producing botulism toxin (low acidity, no free oxygen in the oil, and warm temperatures).
To store your roasted garlic, you can:
1. Leave it in the bulb instead of squeezing the cloves out. Just wrap it in foil and it will keep for a few days in the fridge without drying out.
2. Squeeze the cloves out and put them in a small jar with enough olive oil to cover them, they'll keep longer this way than wrapped in foil.
3. Smash the cloves using the side of a knife and mix with a pinch of salt to make an even paste. Combine this with some softened butter and some fresh or dried herbs (basil and oregano or tarragon would be nice, also lemon juice and/or zest). This can then be formed into a log and wrapped with plastic wrap and put in the freezer. This type of compound butter is great on toasted bread, stirred into rice or pasta dishes, added to a sauce, or added to fish or vegetables when steaming in foil pouches.