No it has to be one or the other. "At 12 o'clock" or "At 12.00P.M" 12:00 = correct to match the display on digital clocks. "at 12 o'clock could be 12 midday noon or at 12 midnight. you will see people writing 12.00 pm/am or 12:00 am/pm it is commonly displayed as : 12:00 on clocks and on public clocks typically at transport hubs, train stations and airports etc In the UK. You will have to ask teachers here what is considered the correct format. I always follow that format because with the military or digital system the A.M and P.M is redundant. 12:01 and over is obviously after 12 noon twelve midday and not yet 12 P.M midnight. So at 2 o'clock or at 2 a.m 2 p.m to distinguish morning before noon(A.M) and night time after 12 noon (P.M).