Why a resistor is connected in series with output of IC's?
I saw an oscillator and the output clock is connected in series with a
resistor. I want to know the use of this resistor.

More than likely (or even almost certainly) it's to stop reflections. So the output impedance of the chip might be around 20 ohms and, when added to the 33 ohm, the net driving impedance is about 50 ohms. So, if the person designing the PCB has done his/her job correctly and chose the track widths to correspond with a transmission line impedance of 50 ohms, then whatever receives the clock signal (at some distance across the PCB) can be assured that the incoming clock amplitude will be regulated to logic levels reasonably well.
Of course, it's quite possible that the clock chip data sheet tells you to do this so, take a read and see what it says.