Concatenate similar SQL transaction rows into single

I have a table of order #'s and hold flags for those orders. I'd like to group the orders into a table/view with a single row and a field containing the flags for the order.

Example
Transaction list of orders
OrderNumber      Flag
Order1                  WC
Order1                  STOP
Order1                  OK
Order2                  WC
Order2                  OK

Group Result
OrderNumber              Flags
Order1                        WC,STOP,OK
Order2                        WC,OK

How is the best way to do something like this?

Thanks