PC Magazine - AOC U2868PQU Review
DisplaySpecifications – 28" AOC U2868PQU - Specifications
That appears to be a 28" monitor with a resolution of 3840x2160 pixels and a refresh rate of 50 – 76 Hz. It has DisplayPort, HDMI, DVI, and VGA inputs. I'd assume that the latter two don't support anywhere close to the full resolution of the monitor.
MacBook Air (11-inch, Mid 2012) - Technical Specifications - Apple Support
That Mac has a Thunderbolt 1 port with a Mini DisplayPort connector that can double as a Mini DisplayPort. The easiest way to connect it to the monitor would be with a Mini DIsplayPort to DisplayPort cable.
However, your Mac doesn't support 4K monitors. It "simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on an external display, both at millions of colors. If you can get a picture at all, it will be based on the Mac's and the monitor's willingness to use a lower resolution, like 2560x1440 (low resolution) or 1920x1080 (which might have fewer artifacts but which would result in rather large text).