For maintaining a healthy workflow environment, the "Purge Order Management Workflow" concurrent request should be scheduled regularly. 
Purge Order Management Workflow Concurrent Program purges workflow items associated with Oracle Order Management, such as OM Order Line and OM Order Header. Unlike the Purge Obsolete Workflow Runtime Data program provided by Oracle Workflow, the Purge Order Management Workflow program purges workflows of closed sales order lines even if the sales order header is still open. It tries to close open workflows in a few very specific cases to make more workflow items eligible for purge. Use this purge program to reduce the size of workflow tables and improve high volume order processing performance.
Information on the use of "Purge Order Management Workflow" concurrent program, to purge closed workflows, specific to Order Management can be found in document: How To Purge Order Management Workflow, Doc ID 878032.1.