Thanks Marco, this is actually what I meant by "purging": deleting the records that are no longer synchronised with the actual state od the DB.
So, if I inderstand correctly, there is no other way than recreating all the schemas and table?
This is quite annoying cause I even have the opposite case now: a table that exists in Oracle (even has a shape column) but that is no longer displayed in ArcGIS... The problem probably arose while I had to delete and re-create this table a few times (sometimes within Oracle, sometimes within ArcGIS). If I rename this table, it appears in ArcGIS. But as soon as I give it its real name back, it disappears... My assumption is that there must be a record in the ArcSDE schema saying that this table is invalid or something...
Any idea?