Oscilloscope is described as 'FOSS oscilloscope app, available on f-droid'. There are eight alternatives to Oscilloscope for a variety of platforms, including Windows, Mac, Linux, Android and VSTHost. The best alternative is Friture, which is both free and Open Source. Other great apps like Oscilloscope are Spectroid (Free), Signalizer (Free, Open Source), Audizr (Free) and Spectralissime (Free Personal).
Real-time audio spectrum analyzer with reasonable frequency resolution across the the entire frequency spectrum. - Spectrum - Waterfall - Max-hold trace - Peak markers - Linear or logarithmic frequency scale
Signalizer is a all-in-one signal visualizing package with a bunch of unique focus-points; real-time audio visualization with optimized 3D GPU graphics, everything being scalable and zoomable gridlessly as well as being arbitrarily precise in both settings and display.
iSpectrum is an easy to use audio spectrum analyzer that allows you to view live audio in a standard frequency plot, a stereo oscilloscope view and a waterfall display. You can adjust the display resolution, center frequency and save images to disk.