Oversampling takes the sampling rate and multiplies it by a factor of 2^n. so 2, 4, or 8 usually. The purpose is to aid in digital filtering only, to allow a different filter to be used so the garbage produced from this digital filter is now further from the audio band. This means softer and less agressive analogue filters can be used on the output of the DAC and IMHO it produces better sound. Although there are plenty of people on this board who for whatever reason grasp onto their antiquated designs for fear of death lol.
Upsampling on the other hand takes any sampling rate and produces any other sampling rate. The most common numbers are 96khz or 192khz, neither of which is a multiple of 44.1khz sound that CDplayers produce. This is a lossy process. The original signal is destroyed in the process by mathematical approximation of what values the new samples are supposed to be.
By contrast Oversampling only adds values with approximation, the originals are unchanged. Oversampling tends to sound very similar in most implimentations, whereas Upsampling can sound very good (Simple Rabit Code resampler), or bad (Foobar's PHSS resampler).