Not recommended, as you could damage the built-in CPU/FPU. If you want it faster, you should seek out a 3640 (or 3660 now).
It can be done. My 16 is OC'd to 25. I have a socket to switch the oscillator back to the original one, and, I have a large heatsink and fan on the CPU and FPU.
HOWEVER, I also have a CPU card in this computer. I HAD to OC the motherboard because X-Surf-100 card requires that you have a Buster-11 chip. The problem with Buster-11 is that it does not work properly at 16MHz, as it was designed for 25MHz. (this was all explained to me by Jens Schoenfeld, so, I take it to be correct). It was true for me, because my x-surf-100 would NOT work on my 16MHz A3000. Even when jumpered into Z-2 mode. I sent it back to Jens in Germany, and, it tested good there, until he tried it on his stock A3000/16. He told me the only solution was to OC the motherboard to get it to work right (or buy another A3000 @ 25 MHz), EVEN THOUGH I HAD A CYBERSTORM MK2 in the computer already, with an 040 @ 40MHz! Well, once this modification was made, everything worked correctly.
So, yes, OC'ing even with a CPU card present will likely be necessary if you plan to purchase an X-Surf-100 card for this computer in the future.