What exactly does "(OC)" mean in products description

What is meant by "OC" (Overclocked) varies depending on what exactly the vendor means. Yes it is ambiguous, and the only way to know for sure is to contact the manufacturer or vendor and obtain clarification. There is no "standard" way for it to mean one specific thing all of the time.

It could mean:

• This chip has been tested to support (under certain assumed environmental conditions) an overclock up to this value, but the default clock speed is the stock speed.
  OR
• This chip will operate at this overclocked frequency out of the box with no user intervention, because we have tested this speed, determined it is safe, and enabled it in the firmware by default. OR
• This speed is something we came up with out of thin air as what might be attainable as an overclock speed under certain circumstances, e.g. extreme cooling solutions.
  OR
• This product has been modified specifically by our manufacturer or vendor to contain special thermal characteristics that are not part of the stock OEM model, which allows the product to safely operate (based on our testing, or not) at this rated speed.

It could really mean anything, but syntactically, "OC" expands to "Overclock" or "Overclocked".

OC means overclocked.

All currently existing CPUs that are compatible will the Z77 chipset (e.g., the Core i7-3770K) only support DDR3 memory up to 1600 MHz.

You can use faster memory, but this would involve overclocking the CPU's memory bus.

OC in supported memory section of motherboard specifications means that JEDEC (the organization making standards for SDRAM industry) does not have an official standard for the speed this motherboard is claiming to support. So this speed is only achievable through overclocking.