Oversampling and Aliasing I am really confused about oversampling and upsampling. I know that upsampling means stretching our signal in time domain by a factor of K, then interpolating some values or using just zeros. As I found out stretching in time domain is shrinking in frequency domain by the same rate. For example, stretch by factor of 2 in time domain mean shrinking by factor of 2 in frequency domain.

Let talk about oversampling, we just increase the sampling frequency more than Nyquist rate. Then we have too much information. Is there any chance to have aliasing? I mean in time domain, lets say I am sampling my signal (frequency is 70 Hz) by sampling frequency equals to 280 Hz (Nyquist*2) so, is there aliasing in frequency domain? Sorry if my explanation is not clear. It is because I am bewildered with these concepts; oversampling, upsampling, downsampling, undersampling, critical sampling and aliasing.

theoretically, I know them but when it comes to practical view, I have serious problem.
When upsampling, you don't really stretch the signal in time. You insert new samples between the existing ones, without modifying the times at which those samples were taken. One property of upsampling is that the waveform remains exactly the same before and after the process; you just increase the number of samples. Note that the main difference between oversampling and upsampling is that the former occurs at the time of sampling, and the latter occurs after sampling has already been done. If there is no aliasing, in theory both produce the same result.

Aliasing is only present when there is a signal at the input of your sampler whose frequency is higher than the Nyquist frequency. In your example, if you sample a 70 Hz signal at 280 samples per second, you will not have aliasing. However, in practice you will not always have precise knowledge or control over the signal you're sampling. For example, you may want to sample a signal coming from an antenna. You don't know in advance what signals are going to be picked up by the antenna. In a case like this, one approach is to low-pass filter the signal before sampling, to ensure no aliasing will happen.