A code practice oscillator or “CPO” is a simple tone generator that is turned on and off with a telegraph key. The telegraph key can be any straight key, or hand key, or semi-automatic key (bug).
A CPO cannot be used by a paddle, which is designed to operate an “electronic keyer,” not a CPO. An electronic keyer generates one or more dots when the left paddle is pressed and one or more dashes when the right paddle is pressed. If you do connect a paddle to a CPO, both left and right levers will act as straight keys. This is something you want to do only if you are using a single-lever paddle as a cootie key.
Sometimes you can connect the output of an electronic keyer to a CPO but be sure to check that the polarity, voltage, and current of the key line on your CPO are within the limits of your electronic keyer output.
Not all CPOs are created equal. The least expensive and easiest to build use a simple LM555 IC as an oscillator, and the resulting square-wave output can sound a bit harsh. Also there may be “key clicks” or other spurious emissions which can cause problems with (for example0 a repeater’s control circuitry. A sine-wave oscillator sounds much better and haS no bad effects on other equipment, but a sine wave is not actually the best for listening. A shaped side-tone oscillator will have a slightly more “mellow” tone than a sine wave, and is more pleasant to listen to.