I like this shot the best:
I don't have a lot of advice, 'cause as I said, you are at a place where I am not further along than you. One thing I notice with Ollie is that he seems quite braced in his neck. It just looks very straight and stiff, right up to the poll area, where it breaks. It's good that he is not breaking several vertebrae back, but laterally, it looks very stiff.
I wonder if working some on jaw flexions would help loosen him. And maybe some old fashioned , cowboy style disengagement of the hind quarters.
For the jaw flexions, there is some info about it on Phillipe Karl's books. He uses flexions on the ground, with the horse in the bridle. And same thing from the saddle.
And as for disengaging the hindquarters, this should start with the hrose
1. Flexing in the jaw/poll,
2. Following the bit around so there is bending in the neck, actually overbending. Be sure that the horse's head does not tilt ( meaning he rotates his head so that his chin comes upward while his ears go off to the opposite side) the face should stay as vertical as possible, even as the head comes around toward the horse's hip.
3. The horse should stop stepping around in a circle, front legs become still and inside back hind steps under, so that hindquarter "disengage " from the "track" of pushing that they normally are following , as the push the horse forward.
So much easier to DO than to READ.
But, it can soften a horse a lot; through the jaw, the neck and the torso and hip.
I would not do it endlessly, as it can frustrate a horse that is trainded to go forward, but one can do it to soften the hrose and remind them to follow the rein and to flex to the inside on the circle. (flex with the jaw to the inside such that you see the jowl kind of tucked into the neck)
ETA I know Phillipe Karl is thought to be a wierdo by some, but the flexions I am talking about are not "his" but come from very old excersizes from classical French school, I believe.