Hi, all the parts that make the up the rear half of Optimus' fire truck are not part of Omega Prime. However, there ARE ways to combine them creatively.
For instance:
You can transform the ladder/head/chest so that it looks like a giant battle axe which Omega Prime can hold.
You can attach Super Mode Optimus Prime's arms to Ultra Magnus' elbows and form over the shoulder rocket launchers.
You can combine both of Super Mode Optimus Prime's feet to make a shield that can attach to Omega Primes arm via peg and hole.
Lastly, you can attach Super Mode Optimus Prime's Shoulder armor to Omega Primes legs to bulk up the armor.
This is how I have my Omega Prime displayed, and he looks kickass.

also, I should note that you have the rocket launchers attached upside down. There are square and round pegs in the teal part (on top where the missiles are). The pegs you are currently using are meant to attach in front of Ultra Magnus.