Best advice... Do not get greedy.
How well you can oc a i7-8700K depends on your luck in getting a good chip.
as of 3/22/2018
What % of I7-8700k chips can oc
at a aggressive vcore near 1.4 or so and delidded
4.9 99%
5.0 88%
5.1 54%
5.2 22%
Note the "delidded" caveat above.
delidding, a $40 process gives a 8700K better heat dissipation capabilities that you likely do not have.
Yes, overclocking is as easy as increasing the all core multiplier in the bios.
Do so gradually and stress test.
Monitor your vcore(read with cpu-z) and your temperature(HWMONITOR or similar)
Ultimately the safe limit for overclocking comes from the voltage(vcore) that you and the chip can tolerate.
That tops out around 1.4v.
Keep a stress test to 85c.
Do not worry too much about this. The processor will throttle or slow down if it detects a dangerous temperature.
That is around 100c.
Again, find you limit and back off some. 8700K is already a beast at stock.
How much do you really need??