If I understood correctly, you are measuring the 16MHz signal with the 1x probe, is that so? If so, think about what is between the circuit and the oscilloscope itself: the probe. It is probably the limiting factor that is distorting your 16MHz square wave. Can you retry with the probe at 10x? It will usually have much less capacitance and therefore much more bandwidth.
Another significant factor that influences this is the location where you are probing - if you have very long connections between the board and the probe points, they will influence the measured signal that arrives at the probe itself. Although 16MHz will not be severely limiting, it might still show distortions if the distance between the alligator clip and the probe tip are very far apart. It might be worth using the small spring accessory that was supplied with your probe. An interesting video is:
As a side note, don't forget that a square wave is never perfectly drawn, since it requires an infinite bandwidth to be fully represented, but quite a reasonable approximation can be done with a bandwidth of about 9 times (10 times is a good rounded approximation) of the fundamental frequency - in your particular case, that would require a 145MHz of bandwidth, which is still within the limits of the oscilloscope (assuming it has true 150MHz of analog bandwidth). The reference below contains some interesting aspects about this:
https://www.allaboutcircuits.com/technical-articles/what-is-the-fourier-transform/Good luck!
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