When your message is exactly the way you want it, you will need to duplicate it for the rest of the contacts in OCA GROUP2. This is easier than it sounds.
Select Compose to open another e-mail composition dialog.
Select Bcc as before, then select OCA GROUP2 this time.
Highlight and Copy the Subject from the first e-mail dialog.
Paste it into the Subject line of the second e-mail dialog.
Click into the Message Area of the first e-mail dialog.
Use Ctrl-A to Select All, then Ctrl-C to Copy the contents.
Click into the Message Area of the second e-mail dialog.
Use Ctrl-A to Select All, then Ctrl-V to Paste the contents.
(NOTE: It is not necessary to delete before you paste.)