You swing OTT because your shoulders roundhouse on the downswing. Your right shoulder comes around to the ball rather than going down as it should do. Quite often this can also be a result of a poor backswing pivot so first things to check here are...

1. Dont overswing and collapse your left arm/wrist
2. Dont slide/sway. Make sure you turn only. No lateral movment backwards
3. A flat left wrist at the top
4. Make sure your shoulders are turned and facing the target
5. Maintain the spine angle and right knee flex and feel coil in the right leg

You need to find a method to come from the inside that works for you. There are lots but a good one to 'feel' the motion is the one taught by John Dunigan. With your shoulders fully turned pull the butt end of the club at the ground. DONT turn your shoulders as you do this and keep your head still (behind the ball)...allow the downward motion of your arms to turn them for you, late in the motion. If you have coiled properly on the backswing the lower body will move forward naturally as you do this (as it springs back from the coil .. this is key) and you should feel like your right shoulder is going down, the left shoulder up (rather than around) and you increase your body tilt away from the target. Keep pulling down and see the club come around square

To you this will feel like you are going to hit the ball way right and it will feel very 'armsy'. You may start off this way but keep a light grip and feel the hands turn over through the ball. So lead with the arms and dont throw the upper body through the ball. Pull the clun through the ball rather than 'hit' at it (which is everyones natural insticnt)

If the ball keeps going right you may have a weak grip. Work on moving both hands to the right (not just the left hand)

Try it a LOT in slow motion and then with half swings with the ball on a tee. You'll know immediately if you roundhouse those shoulders and come OTT as you will hit a pull or a slice

HTH