I've relied on Z'dar and his earthquake skill a lot. The first two times I got party members to level 30 I was mostly playing with Tiberius, Tlanac, Thia and Z'dar. I seriously don't think I could have gotten to the end of several of the harder fights without Z'dar.
Today I bought O'da and find that he compares favorably. This last time thru the game I've been using Fel'mor and find that his taunt seriously lowers the need for earthquake so I've been able to drop Thia and add Kassia. Now I'm back and fourth as to using O'da or Z'dar.
The way I see it they both deliver good regular damage with their basic attack. Z'dar has a much better AC but I think that's because he has much better equipment. The skills break out fairly evenly as well.
I see bombastic as similar to Immolate. Both cause a lot of damage over a fairly long time but bombastic hits harder up front and can affect multiple enemies. The problem is I can't send the bombs to an area, instead they center around an enemy. Advantage to bombastic
Sniper shot and Pyroblast both deal big damage but here it's Z'dar's skill that affects multiple enemies. However, the blast does a lot less damage. O'da can hit for 4000HP!
Oda's jetpack stuns enemies for five seconds while Z'dar's earthquake stuns for six. Jetpack does about the same damage but earthquake only hits once and jetpack hits three times. Earthquake however stuns the whole battlefield. In my book that outweighs the extra damage.
The last thing to note is that O'da is really small. He can be hard to select if he's in a crowd and for this reason I recommend you not put him in slit 2 of the party. On the other hand he's really fast on his feet.
What do you folks think of O'da?