'Strength' for Nietzsche' is strength to choose our own values. This strength is lost, or never possessed, if we have been infected by 'slave morality' as he calls it. Slave morality prescribes what we must do; the strong for Nietzsche are by contrast the self-affirming, who reject all idea of what we must do, and create their own values. The creation of one's own values is the mark of self-afffirmation and hence of strength. ('Genealogy of Morals', I:2.)
This is clearly a special sense of 'strength'. The strong are not those who subjugate others, who coerce the weak. And the weak are not those who lack the physical power or the psychological ability to stand up to the strong. Nietzsche is working with an unconventional understanding of strength and weakness. When he praises the strong he does not understand strength in the same way as, say, Thrasymachus in Plato's 'Republic'.
Given this special sense of 'strength - and correspondingly of 'weakness' - it is hardly possible for the strong to oppress the weak and to abuse their own power. If I choose my own values, in which my strength consists, I can scarcely (simply by virtue of this choice) oppress the weak - whose weakness consists solely in the fact that they do not choose their own values.
Some mention should be made of compassion, on which Nietzsche appears to be particularly hard : 'compassion I recognized as more dangerous than any vice' ('Will to Power' : 54). But in my view what Nietzsche mainly has against it is (a) that it distracts the strong, by its psychological pull, from choosing their own values (they are dragged down by this one); and (b) that it treats others as mere passive objects in need of help. By extending compassion of them, the strong are not helping them to become conscious of themselves as potentially self-affirming, value-creating agents ('Beyond Good and Evil' IX : 270).
For all his brilliant style, Nietzsche is an extremely difficult writer to reconstruct with any assurance. I have offered a reading of the relevant part of his work. It is serious but tentative. No sympathy on my part should be assumed for any of the views I have attributed to him.