Significant low frequency content from digital oscilloscope measurement
You state you've not done any windowing, even if you have, you'd still expect a large DC signal (from the vertical offset of the trace), which will have leaked into other low frequency bins.

The FFT takes a signal which is a loop (ie it repeats over and over again). If your signal doesn't match at the ends, you get frequency components that correspond to harmonics of the period of the loop. There are various techniques for helping fix this, such as windowing, zero padding and pre-whitening. 

If you're genuinely getting a signal at that frequency (eg most things have 50 or 60Hz components from AC), you can remove it, by filtering, either in the time domain before the FFT (using a FIR or IIR), or afterwards in the frequency domain after the FFT.