I think you need to replace this:
Outlook._MailItem oMailItem = (Outlook._MailItem)oApp.CreateItem(Outlook.OlItemType.olMailItem);
with this:
Outlook.MailItem oMailItem = oApp.CreateItem(Outlook.OlItemType.olMailItem);
Also, if this code is in an add-in, you don't need to create a new instance of Outlook.Application - use the object passed to the OnConnection event.