Well for starters, what are your intentions for this youngster? He is pretty soundly dressage bred - although there is some jumping blood in there, he is not what I would consider well bred for jumping.
I'm afraid I don't know much about the dam line but I have met several Sandreo youngsters.
Osmium did not compete much due to injury as a young horse. His pedigree is full of old Dutch lines that tend to produce the older style horse. Lower to the ground, with more bone and a heavier look. In general, easy to train and comfy to ride.
Sandreo is marketed as a dressage sire (although there is some very good jumping blood in his pedigree), and is touted as a refining stallion. He tends to throw elegance, long legs, and a nice head. His sire, Sandro Hit, has some jumping blood but has made his name producing top level dressage horses. He is a horse that tends to put a bit of fizz in his offspring, and some will say that in general Sandro Hit does not produce ammy horses. Flemmingh (dec) also has some jumping blood but is regarded as a dressage sire. He was well known to produce easy going youngsters with very correct paces and good limbs.
Sandreo youngsters seem to be fairly level headed - certainly the ones I have met are all lovely natured and quick, easy learners. I have not seen one under saddle as yet - the oldest one I've met is only 2.
The Sandreos I have seen have been very beautiful and leggy with super natures and light, effortless movement (even those out of heavier old-school Hano mares). They have a bit of spark and definitely enjoy being centre of attention but did not seem difficult or pushy. He is a stallion I would definitely consider using if I had a chunky old fashioned style mare.
I'm sure there are others on this BB that know a whole heap more about this than I, but it's a start