Revit MEP supports oval duct, but the symbol in tags my firm has used since hand drafting is the Greek letter PHI. Unfortunately there is no key in for the PHI character. I can however, use the "Select" and "Copy" buttons to copy the character to the clipboard and paste it directly into a text string.
In this case, the symbol legend.
It can also be pasted directly in a label for a tag as a suffix.
Happy New Year!