There are several articles available on the web that explain this so I won't go into great detail.
Oversampling is used to improve digital filtering.
Upsampling can mean several things...in the case of audio DAC's, it generally means multiplying the native sample rate AND increasing the bit rate from 16 to 24 bits etc.
A mathematical algorithm is used to create what it thinks a 24 bit version of the original 16 bits would look like. It is not possible to "make-up" a new signal more accurate than the original 16 bits.