What would a venial sin of omission be?
A sin of omission is when you neglect to do something that you ought to do.
A sin of comission is when you do something that you should not do.
What makes it venial or mortal is the same in either case: the seriousness of the sin, your degree of awareness that it was wrong, and the degree of your full consent to it.
a sin of omision might be to maintain silence while you watch someone be unjustly accused in a minor matter.