In Oracle APEX, when creating a form on a table using the wizard a "Process Row of <table>" process is generated for the page which will handle the inserting, updating, and deleting of a record. The default message is "Action Processed". What if you wanted to have a different messages depending on if you where inserting, updating, or deleting? You could remove the wizard generated process and create a manual process to perform each of the DML optiosn and return different messages.
To use only the declarative feature of APEX it may be simpler to just create multiple "Process Row of <table>" processes to handle each DML method individually. In the "Process Row of <table>" process there is a field in the "Source: Automatic Row Processing (DML)" region called "Allowed Operations". The field has three checkboxes indicating the DML operations allowed: Insert, Update, Delete. There is also the "Process Success Message" field which contains the text of the message that will be shown when the process successfully executes.
To create a different message for Insert, Update, and Delete copy the default "Process Row of <table>" process twice. You may want to name the process to describe the DML process you will be setting it to. After making the copies of the process ensure that they will be executed before any other process that may change the session state of the page items (i.e. before a "reset page" process).
Edit each process and set the "Allowed Operations", "Process Success Message" and "When Button Pressed" fields to reflect the DML action and message to be shown for each operation.
Make sure to set the "When Button Pressed" condition to match the button with the allowed operation. Failure to set this correctly will result in an error (Invalid action <ACTION> on this object ...) when you try to do an action specified by the button but not allowed in the process.
Example page is here