Stable ±15 V symmetric power from USB
I'm surprised that this even works, even more surprised that you claim it works okay.

Here is the datasheet for the ICL7660. Let's take a look:



Your 330 mA is way off the charts. The first trick to knowing that it's not usable for much is that they don't even mention the maximum current draw on the front page of the datasheet.

The reliable way to get a symmetric ±10 V with more than a couple of mA output is to use a switch mode converter. There are a few topologies to chose from, and a lot of simple ICs to use.

Now, to your oscillator question. The datasheet shows that you can lower the oscillation frequency by connecting a capacitor to the OSC pin:



Buried in the details is that you can also increase the frequency, by driving the OSC pin directly:



Increasing the frequency may very well give you a lower output ripple, at the cost of conversion efficiency. The datasheet also mentions that low ESR capacitors will help. I have a feeling that you're seeing excessive ripple because the device is simply not designed for these currents.