Special thanks to user Jamesie
flashing your Omnia is possible in windows 7 XP mode and is really easy to do - below is a modified version of sectors tutorial for standard XP with a few additional steps (In bold) required to flash in windows 7 XP mode
Flashing in windows 7 /XP mode
1. Install Windows XP mode for free if you have windows 7 from:
2. start virtual XP in full screen mode
3. Install Microsoft ActiveSync 4.5 (first time use only)
4. Install the Samsung Modem Driver XP (first time use only)
5. Reboot the virtual XP to full screen
6. After these first steps try to disconnect your Omnia from USB and connect it back on using the "Attach i900" in the USB menu at the top of full screen virtual XP mode. Repeat it 1-2 times so the PC can recognize the phone.
It is advised to reboot your virtual XP back to full screen. (first time use only)
7. Make sure that you have a backup of all your important stuff (contacts, pictures etc).
8. Switch OFF the phone by holding the Power button for ~5 seconds.
9. Run the updater. (Works the same way for Original or Cooked ROMs)
10. Switch ON the phone again by holding the Power button.
11. Press "attach i900" in the USB menu at top of full screen virtual XP when you hear device attached sound
12. Press START on the updater and your phone will start flashing. Do not move your phone or the USB wire as it might fail during flashing
13. after a few seconds the phone reboots - Press "attach i900" in the usb menu at top of full screen virtual XP when you hear device attached sound and flashing will continue.
14. Wait until the updater shows JOB DONE! DO NOT DISCONNECT BEFORE.
15. Disconnect your phone.
16. Leave the phone to reboot 2 times, calibrate screen and finish the start-up procedure.
17. Enjoy the new firmware.
pretty straight forwards, I hope this helps, but as always flashing is risky if not done properly and i don't accept responsibility for mishaps