Atom comes with 3 separate Oscillators, which each can create their own waveforms. The three oscillators (OSC 1, OSC 2 and OSC 3) are located in the center of Atom's interface:

The shapes shown on the left hand side of each oscillator section determine which waveform the oscillator creates. The top option is a Sin wave, below that is a Triangle wave, followed by a Saw wave in the middle. The next waveform is a square, and the bottom waveform is a noise waveform. 

Each oscillator can use a different waveform, and the waveform currently selected will be highlighted. In the below example this sound uses one saw wave, one triangle wave, and one noise wave:

To turn an oscillator on or off simply turn the "Volume," knob for each oscillator above 0, or conversely click the light shown at the top right corner of each oscillator section. If the light is lit up pink it means the oscillator is turned on, but if the oscillator light is grey then that oscillator is turned off. In the below example Oscillators 1 and 2 are turned on, but 3 is turned off:

The "volume," knob of each oscillator controls how loud (in dB) the oscillator's sound is. Turning this knob to the right will make this oscillator's output volume louder, while turning it to the left will make the output of the oscillator quieter. Turning the knob all the way to the left will turn the oscillator off entirely.

The "Pitch," knob will adjust the pitch of the oscillator, with each number representing semitones. Turning it to the right will raise it in semitones (a Pitch of 9 means that oscillator's sound is 9 semitones above the currently played pitch for example), while turning it to the left will do the opposite and lower the pitch the oscillator outputs. 

The "Fine-Tune," knob adjusts the pitch by a fraction of a single semitone. This means the most it can raise or lower the pitch is by nearly a full semitone, and this control can be useful for making wide supersaw typed sounds. 

"Hi-Cut," applies a filter that removes higher frequencies in the oscillator's sound. The more the knob is turned to the left the more the high frequencies in the sound will be removed. If the knob is turned fully to the right then no high frequencies will be removed.
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"Lo-Cut," applies a filter that removes lower frequencies in the oscillator's sound. The more the knob is turned to the right the more the low frequencies in the sound will be removed. If the knob is turned fully to the left then no high frequencies will be removed. 

The "Voices," section dictates how many instances of the oscillator are used for a given sound. To raise or lower the number of voices click your cursor over the voices box, and drag it up or down to raise or lower the number of voices:

For example, if we set OSC 1 to "6," in the voices section then oscillator 1 will be duplicating its effect 6 times. This can be useful for stereo sounds like supersaws.

The "Detune," parameter slightly changes the Fine Tune of each voice used in the oscillator. Some will be slightly louder in pitch, while some will be slightly lower. Turning the knob fully to the left will not detune the voices at all, meaning they will all be the same pitch exactly. Turning this knob to the right begins to adjust the pitch of each voice in the oscillator. 

The "Stereo," control helps place the oscillator's sound into the stereo field, making it sound wider when turned to the right. When turned to the left completely the sounds are not panned at all.