Dave had it covered but yes Onachus is the list suited for the old school tyranids. Do post up your feedback and if possible battle reports of your games and that helps shape and balance the list and get it closer to being approved.
Newer Tyranid and Forge World units have gone a different direction instead and these have been represented in the Leviathan list, which also has different special rules and has gone a different direction.
The rulebook had various errata released over time and the Epic community have over time rebalanced the lists to fix internal balance issues (the rulebook had a number of Space Marine, Imperial Guard and Titan formations that were found to be under or over costed). The Ork list is virtually unchanged but I would still generally recommend getting a copy of the
2013 Tournament Pack (basically the EpicA rulebook including the errata changes plus FAQ below on each page, along with the army lists balanced enough to be declared 'approved' by the community) printed out and bound, or have it on a laptop near by.