The Greek translation of Matthew inadvertently dropped the word falsely from the Hebrew Matthew. This erroneously made it appear Jesus said one is never to take an oath. (Nehemiah Gordon, Hebrew Yeshua v. Greek Jesus (Hilkia Press, 2006) at 59, 65-66, 68.)
But God commands people to take oaths in God's name. "Thou shalt fear YAHWEH thy God;... and by his name shalt thou swear." (Deu 10:20 ASV.)
Gordon, a Jewish scholar, notes the Pharisees evidently taught you could violate an oath as long as not sworn in Yahweh's name. In other words, false oaths were accceptable to them, as long as God's name was not brought into the statement. This was based upon twisting the Bible which prohibited any false swearing in God's name. (Lev. 19:12.) But would false swearing truly be OK if God's name was not invoked? Not likely.
Jesus' criticisms imply the Pharisaic quibbling with Lev. 19:12 led the Pharisees to sanction false oaths as long as not in God's name. Implied from Jesus' criticisisms is that the Pharisees obviously said Lev. 19:12 meant one could falsely swear even if you invoked objects closely associated with God, like the Temple. You supposedly would transgress the command only when God's name is actually used.
However, Jesus was invoking the broader principle in Zechariah 8:17 which said "love no false oath: for all these are things that I hate, saith YAHWEH." Thus, you were not allowed to dupe others if you worded your oath carefully. Thus, the Pharisees diminished the Law once more. Gordon detected the difference in the Hebrew version of Matthew (i.e., the Shem-Tov) where Jesus corrected them, saying `do not swear falsely at all,' whether by the temple or anything else. The Greek translation inadvertently dropped the word falsely. This led us to misapprehend Jesus' meaning.
Then Gordon explains the instruction ending `anything beyond this is evil' was an Hebraism used in the Original Testament to mean that anything beyond (added to) the Torah was evil.