Oil is a "must have" resource at the start of the modern era, and if you don't have any, or it is located in a bad location, you may need to change your overall strategy slightly. The AI civs don't like to trade Oil on the higher difficulty levels, and often don't have any spare Oil to trade anyway. As such, it's unwise to rely upon foreign trade to supply you with Oil, and if you do, then you are wide open to the AI cutting it off at just the wrong moment.
The two worst locations for Oil are right on the edge of your borders with a neighbouring civ, or offshore.
You need a large and mixed naval force to defend an offshore platform, and battleships alone are inadequate as they can't see or attack enemy subs, so you need destroyers (which can see / fight subs), plus battleships, (which can fight off enemy surface ships). You also need nearby fighter cover, and a few cruise missiles and/or bombers nearby in case you have to soften up a huge stack of enemy naval units.
You never know in advance if your civ has any Oil, or where it's located, so it's a good insurance policy to plan on it being offshore. Locating national wonders like Heroic Epic / West Point / Iron Works in your most productive coastal city give you an insurance policy, as does settling a few Great Generals in that city along with a Military Academy.
You will then be ready to quickly churn out strong naval units that are already promoted, and will be able to defend an Offshore Platform with a smaller naval force, that won't take as many casualties.
If you find yourself in a situation where you have no oil, or it is located offshore and you are not in a position to defend your Offshore Platform, then like I said you may need a slight change of strategy towards the end game. I have played loads of games whereby I planned to avoid war completely, because my end game strategy was based upon a peaceful victory via space / culture / diplomacy. Such plans often ended up in the trash bin upon discovering I didn't have any Oil, or it was offshore, and bloody difficult to defend because my coastal cities were very non-productive. Such scenarios usually ended up with me having to go to war to get the safe supply of Oil that I needed.
These days I make sure that I always found or conquer at least one highly productive coastal city, as that one city can easily defend an offshore platform if you feed it the items I mentioned.
Regards - Mr P