Dell have three form factors for the OptiPlex 7000 series;
Micro
SFF(or Small Form Factor)
and
Tower
are you referring to the last one linked? In either case, the OptiPlex's are designed for workstation environments whereby said environments need reliability. Why am I saying this, it's due to the fact that overclocking or going beyond what's spec'd by the hardware out of the box will cause instability issues. These can lead to system down time and possibly a bad reputation towards the prebuilt system's maker.
That being said, the models that have DDR4- ram in them will be able to go to DDR4-3200MHz, which is an overclock as the base speeds for DDR4 is 2133MHz. SO yes that's the only arena where you'll be able to overclock. Not the CPU though as none of them come with a K suffix processor.