Oshimaru is in a journey to exhaust and seal evil spirits. They can't be killed but sealed. Because they are causing imbalance in the realm of humans and spirits. After they are sealed oshimaru can use there power but it's highly risky because spirits can escape if the power is overused. So he can't truly master any of this powers. And he has to adapt to every challenges instead of depending on the powers of spirits. Eventually he face spirits that pushed him to his absolute limits.