Of

[Allen165]

"The department provides services for all of the companies of the group."

Leaving out the first "of" would be technically wrong, in my opinion, though I realize that in spoken English it's often left out.

Do you agree?

Thanks!

 

[SoothingDave]

It's not wrong.

 

[emsr2d2]

In fact, the inclusion of "of" in that particular sentence made it sound quite unnatural and awkward.