Document And Entity Information - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2017
Dec. 27, 2017
Mar. 31, 2017
Document And Entity Information [Abstract]      
Entity Registrant Name Oncobiologics, Inc.    
Entity Central Index Key 0001649989    
Trading Symbol ons    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Current Fiscal Year End Date --09-30    
Entity Filer Category Smaller Reporting Company    
Entity Well-known Seasoned Issuer No    
Entity Common Stock, Shares Outstanding   25,530,727  
Entity Public Float     $ 44.5
Document Type 10-K    
Document Period End Date Sep. 30, 2017    
Amendment Flag false    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY