I purchased a 2 pack of Oral-B CrossAction brush heads. I am confident these are fake by the following observations:

1) The coloured ring does not snap on as tightly compared to genuine
2) Oral-B logo is lighter grey compared to genuine
3) Bristles feel different to genuine
4) The bristle insertion mould is different to genuine
5) The individual brush packaging is lighter colour than genuine
6) Packaging thickness is less than genuine (need to confirm this)