Working of Hartley oscillator :
When the collector supply voltage Vcc is switched on, the capictor gets charged. The capacitor discharges through the inductor, setting up oscillations of frequency: -
f = 1/ (2p*[C (L1 + L2 + 2M)] 1/2 )
Considering the fact that there exists mutual inductance between the two coils because the coils are wound on the same core, their net effective inductance is increased by mutual inductance M. so in this case effective inductance -
L = L1 + L2 + 2M.