The GA-Z87X-OC, will only work 2-way Nvidia SLI. But up to 4-way for AMD Crossfire.
Taken from the gigabyte web site:
Support for 4-Way/3-Way/2-Way AMD CrossFire™/2-Way NVIDIA® SLI™ Technology
The GA-Z87X-OC Force, will work up to 4-way for both AMD crossfire and Nvidia SLI.
Support for 4-Way/3-Way/2-Way AMD CrossFire™/NVIDIA® SLI™ technology