Transient behaviors near the oscillation threshold are investigated in a Wien bridge oscillator. Undamped relaxation oscillations familiar in masers and lasers are observed. A burst of oscillations occurs after a delay from switching on a power supply for an operational amplifier used for the oscillator. The shape of the burst closely fits to the hyperbolic‐secant function. Observed results are analyzed by van der Pol’s equation and analogies to the phenomena that appear in the magnetic resonance and laser‐type oscillators are briefly discussed.