Send your tanks East until they find the oil field. Build a well on this oil field, don't worry about the outlying turret and observation post, it doesn't matter if they are lost. Leave the tanks guarding the oil for now, when the Well is complete build a couple of turrets to defend it. Build a further turret guarding your original oil well.
When these turrets are complete start continuous tank production. At this stage you may need to use your tanks to help drive off attacks on either of your Wells.
Once you have a squad of six or seven tanks you want to launch an attack on the computers Oil Wells, the location of these isn't guaranteed, there will probably be a Well on one of the oil field you could just see from one of original observation posts. Otherwise the most likely site is one fairly central, but slightly into the computers half of the map.
There may be attacks on your base any time now, if this happens while your tanks are away on an attack, bring them back as fast as possible, losing a Well at this stage can be very damaging.
If you have now managed to destroy a well or two whilst successfully defending yours the advantage should now be yours. Pick a new oil field to build a well on, I'd recommend the central one as it is a grade (A) well, giving you faster oil production. First stop tank production, then send a squad of tanks to the site, build a pair of turrets, only when they are complete and there is no sign of attacks build an Oil Well.
Once this Well is complete resume tank production, if you have sufficient forces recheck the oil fields for enemy Wells, if they've built any destroy them. If you've made it this far, the game is almost over. Wait till you have a large squad of tanks and mount an all out attack on the enemies main base.