Where is the oscillator in the 555 chip provided here?

Keep in mind the schematic shown in the datasheet is probably not exact. Think of it more as a conceptual description.

Q1-Q6 and Q7-Q10,Q12-Q13 are each differential inputs, the front ends of comparators. Notice that one side of each comparator is brought out and the other is either at 1/3 or 2/3 of the supply. Q15-16 is a current mirror. The way it is hooked up it provides a lot of gain to the left comparator. The stuff right of that is mostly logic to produce the right signal in the right cases and to control the discharge output. The stuff way at the right is the main output pin driver.

It's probably simpler to explain by showing how to build an astable multivibrator (like a 555 circuit) from a raw op-amp:

The operation is explained in the source I stole the diagram from.

But basically, the op-amp is operating as a comparator. Assume the output starts out high (V6 ~ Vcc), but capacitor C1 is discharged. The output will stay high as C1 slowly charges through R3. When C1 charges up to Vcc / 2 (the value set by the R2-R1 divider), suddenly the (-) input is higher than the (+) input, and the output will go low (to ~Vee, which must be below ground). Now the output will stay low while C1 slowly discharges through R3. When C1 is discharged to Vee / 2, the comparator is triggered again and the output goes back high, and so on.

You can follow through the source link to see how to modify the circuit for monostable operation and a couple of other variants.

Note that in the 555 circuit, R3 and C1 are external components you have to provide, and the circuit is also modified to operate from a single supply.