Observe the base circuit of the transistor - it has two paths. One is the voltage divider which simply turns the transistor ON via the feedback winding (which basically has almost zero DC resistance). The other is via the transformer's feedback winding's flux once the circuit operates.
When power is first applied, the transistor is OFF - the voltage across the lower base resistor turns it ON - at this instant, the feedback winding has almost zero impedance to DC. As the transistor turns ON, flux builds up in the primary winding which is coupled inductively to both the secondary and feedback windings. The feedback winding is such that the current through it opposes the current going into the base, and therefore reduces the available base voltage - as soon as this voltage falls below 0.7 V (the transistor's turn-on voltage), the base diode no longer conducts and the transistor turns OFF.
When the transistor turns OFF, the flux through the primary starts to decay, and so does the flux through the secondary and feedback windings. Once the flux through the feedback winding is close to zero, the voltage across the lower resistor again turns ON the transistor via the feedback coil (which at this moment presents a low impedance) - and the cycle repeats itself (i.e., the circuit oscillates) as long as the power is applied.
Hope that helps. And, BTW 2N3055 is an NPN transistor, not PNP as the thread's title says...
