An OMS answers questions of record. What did the client ask for, who approved it, which account does it belong to, what restrictions apply, what is the current position, and what has settled.
It is deliberately not built for speed. Execution tactics live in an execution-management-system; the OMS holds the parent-order and receives fills back to keep the official position correct.
Example: a portfolio manager enters a 400,000-share buy in the OMS. Compliance blocks 100,000 because the fund's position limit allows 300,000. The remaining parent order is staged to the EMS, which works it as child-orders across venues and reports each fill back for allocation.
Related: execution-management-system, parent-order, fix-protocol, position-netting