You are correct. Omega 3000 was offered with three different 6cyl engines. There was the C30LE with 156 hp and C30NE (and 30NE earlier) with 177 hp. These were 12V uniside engines, just like the first CIH 6's from the mid 60's. Then there was a 24V C30SE engine with a crossflow head that had 204 hp.
Later Omega was also offered with (consumption and emission purposes) 2.6 liter uniside with the dual ram air intake known from the C30SE. There was a further homologation series done for the German DTM series, Omega EVO 500 was the result. The german tuning companies (Irmscher, Mantzel anc co.) offered also 3.6 12V and 4.0 24V versions. And then there was of course the Lotus Omega with twin turbos and 377 hp.
These are the top range of Omegas, the basic models had 2.0 ohc engines and rather crappy interiors. Omega also had a 2.4 liter cih4 engine and it has been popular tuning to cut the 24V cih6-head and place it on top of the four to make it a 16V. Exotic but rather expensive as it needs new cams etc. Opel itself raced 16V CIH4 crossflow heads but they are even more exotic and way more expensive. They were used in rallying in Ascona 400 and Manta 400 models. Now the german tuning house Risse offers the engines and parts.
Omega B was another thing, it no longer had straight-6 gasoline engines as they were replaced by V6's (2.5, 3.0 and 3.2). It still had a 2.5 I6 turbodiesel but that one came from BMW. Omega A also had a diesel, 4cyl that used the CIH block. This was also used in previous Rekords in the D, E1 and E2 generations (2.1, 2.3, 2.3TD).