You cannot get Lugia in Omega Ruby through normal play. It is version exclusive to Alpha Sapphire, against Ho-oh in Omega Ruby.
The encounter occurs at Sea Mauville. In Alpha Sapphire, the "encounter ring" will be underwater in what appears to be the engine room. In Omega Ruby, the ring will be perched high above on some over-turned support structures. You will need to complete a mini side quest to obtain the Tidal Bell (Lugia, Alpha Sapphire) or the Clear Bell (Ho-oh, Omega Ruby). It will be at level 50. Ho-oh also is holding a special item Sacred Ash, while Lugia will hold nothing.
Experience (I encountered Lugia in Alpha Sapphire)