Usually EWCM is during the ideal time to BD, before O.
I start to get it a few days before O, and for me it means I will O soon because by the time I O it is mostly or completely gone. I have heard quite often that O will be right after the heaviest EWCM day, but mine can be 2-3 days later according to my chart.
Anyhow, the CM is there to help the sperm along so no O should not be already happening/past because then it would be too late. (from the TTC perspective)
ETA the most fertile window is generally considered to be three days before O until the day of O. Like HulaJenn said, different women can have different amount of EW over a different number of days and even different cycles can be different for the same woman. From hearing about others' experience an "average" woman may get about 2 days of EW with O the day after day 2.