He certainly is not comfortable jumping those heights--he has no confidence in himself to get over them. That's why you're chasing him so badly, which is leading to awkward distances--and all of these things make your horse think, 'you know what? This jumping thing sucks!'
You want everything to be as easy as possible for him. Free jumping does not mean you raise the bar as high as possible to see how high he can jump in one sitting--it's a gymnastic exercise where he'll calmly navigate the fences on his own. He is not doing this with you chasing him!
First thing is first, bring the height of EVERY jump down. There is no need to put the stress on his legs; he is not in the right shape, nor does he have the confidence, to be jumping those heights.
Work on smaller jumps to build up his own confidence and condition. Do lots of grid work--it will help you get in his rhythm, and work on your position too. If you jump ahead or get left behind or catch his mouth, Oscar will again think, 'you know what, I really don't know about this jumping thing...'.
There are plenty of horses that can jump 5ft but aren't doing so competitively--at that height your horse needs a LOT more then just 'can he jump it?'
I think he has a lot of talent, but don't waste it and don't take it for granted. Take your time, go slow, and above all, keep the jumps smaller.
As I've read before, horses don't get confidence by jumping big jumps--they get confidence in jumping a ton of different small ones. ;)
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