There is no other as versatile as OLGA when it comes to multiphase flow.

But if u intend to work on transient conditions for single phase then Pipelinestudio OR AFT impulse OR Pipenet transient module are OK. Pipeline stoner is another software but I dont have any experience on that.

But slug study, pigging features of OLGA are not available in above three. May be pipelinestoner has it. checkit.

So it depend on ur requirement