Jumping oxers can be nerve racking for everyone, regardless of the height!
If ever I feel nervous i like to think about what exactly I and nervous about.
Is it the height/width? Make it smaller until I confident jumling it, then gradually increase the height.
Make sure you are not looking down into the bottom of the fence, look forward over it. Make sure your pony is in a good rhythm and is going forward from your leg into your hand.
I know this is easier said than done, but often if you are nervous, the fear travels to the horse and they get tense too, which in turn leads to a bad jump. Breath and relax!