A "normal" LC oscillator starts by amplifying noise (which, like the poor, is always with us). The resonant circuit selects the frequency and the signal grows (due to positive feedback) until it reaches the equilibrium level of oscillation (set by the amplitude stabilizing method, which varies from design to design). Another starting mechanism is the shock of applying power to the circuit, which starts an oscillatory voltage in the LC circuit.
Unless it doesn't: a corollary of Murphy's Law states that "self-starting oscillators won't".
One method of limiting the oscillation is merely when the voltage swing reaches the maximum allowed by the supply voltage. Another is a reduction in loop gain as the average bias on the active element increases. Sometimes the output is rectified and used as a control voltage in a DC feedback circuit to the bias voltage. A useful oscillator requires both a frequency-determining network (LC in this case) and an amplitude-determining network, since the oscillation, left to its own devices (sic) would increase without bound in the simple mathematical analysis, and that could never happen.