Board looks ok to me, ready to proceed to next step.
I see tiny silvery flakes on the jointed surface, which I suppose were a part of the knives just moments ago
Your knives may not be good quality! Oak is hard, and knots are harder, but you should not see chipped edges after only 1 light cut. But maybe these are just flakes of "wire edge" from poor sharpening job.
If edges are chipped I recommend adding back bevel to knives to strengthen edge. Alternative is to make cutting angle steeper with second bevel, but then machines will not work as well on softer woods.
Am I doing something wrong, or is it better to leave some boards for the sandpaper?
Are you trying to produce finished surface?? Then yes you are doing something wrong.
Jointer and planer are to produce flat surfaces, not smooth surfaces.
With normal steel cutters any cut, any feed rate and any wood will not give perfectly smooth surface. You must complete job by sanding or scraping or hand plane.