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First, let's understand the four options provided: (A) Positive feedback (B) Large gain (C) No feedback (D) Negative feedback An amplifier can have different types of feedback and gain values, and it's crucial to know how oscillators relate to these features.

In general, an amplifier is a device used to magnify or increase the power of a signal without affecting its shape. Oscillators, on the other hand, are amplifiers specifically designed to generate and sustain oscillation (periodic waveform), which means the output signal isn't only amplified but also characterized by a specific periodic oscillation.

To make an oscillator work, the primary component is applying feedback to the system. Feedback can be either positive or negative. Positive feedback is used to build a desired output signal level by allowing a portion of the output signal to be fed back into the input. This feedback strengthens the oscillation, making the system self-sustaining. Negative feedback, on the other hand, is used to stabilize the gain in an amplifier and keep the output signal level within desired limits. It does this by reducing the overall gain when the output signal increases. No feedback would mean the output is never connected back to the input, and without any interaction between them, it is impossible to build an oscillating system.

Considering all the information discussed, the correct characteristic feature of an oscillator in the given options is (A) Positive feedback.