Why exactly a RLC Circuit powered by DC oscillates?
lets consider R=0 and L and C are ideal.


Just think about a big flywheel driven through an elastic rope by a motor. The capacitor is the flywheel and the elastic rope is the inductor. The equivalent of DC being applied to your LC series circuit is the motor running at constant speed. The torque of the motor is current.

At first, the flywheel barely moves even though the elastic rope has been turned by the motor several times but, after a short while, you see the flywheel starting to rotate but, its rotation speed is much slower than the motor. As time goes by the rope gains more turns and transmits a bigger torque to the flywheel until hey presto. The flywheel is rotating at exactly the same speed as the motor.

But there is still stored energy in the elastic rope and this continues to exert more torque on the flywheel. Now the flywheel is rotating faster than the driving motor and also the elastic rope is starting to lose some of its turns.

After a while longer the elastic rope has run out of energy - no more turns and the flywheel is rotating at exactly twice the speed of the motor. The elastic rope now starts getting wound backwards - the flywheel is rotating faster than the motor and this starts winding turns in the opposite way. This creates a counter-torque that also starts to decellerate the flywheel and once again, there comes a situation where both motor speed and flywheel speed are identical but the energy in the elastic rope is slowing the flywheel down until after a while longer it can be seen to stop it completely. At this point the elastic rope has fully got rid of its energy/turns and the flywheel has no energy due to it being momentarily stationary.

Then, because the motor is still spinning, the process begins again.