Hello New to mechanics!
Yes, it is possible to crit and onslaught a creature at the same hit, as you can see here.
Please notice, though, that the chance of it happening'll be calculated as the chance of onslaughting (the % depends on the level of the item, from 1 to 10) multiplied by the chance of getting a critical hit (that will depend on some factors as runes, imbues and weapons).