Source: http://oradoc.photo.net/ora81/DOC/server.815/a67779/ch2.htm#35921

AD A.D.	AD indicator with or without periods.
AM A.M.	Meridian indicator with or without periods.
BC B.C.	BC indicator with or without periods.
D	Day of week (1-7).
DAY	Name of day, padded with blanks to length of 9 characters.
DD	Day of month (1-31).
DDD	Day of year (1-366).
DY	Abbreviated name of day.
HH	Hour of day (1-12).
HH12	Hour of day (1-12).
HH24	Hour of day (0-23).
MI	Minute (0-59).
MM	Two-digit numeric abbreviation of month (01-12; JAN = 01)
MON	Abbreviated name of month.
MONTH	Name of month, padded with blanks to length of 9 characters.
PM P.M.	Meridian indicator with or without periods.
RM	Roman numeral month (I-XII; JAN = I).
SS	Second (0-59).
SSSSS	Seconds past midnight (0-86399).
WW	Week of year (1-53) where week 1 starts on the first day of the year and continues to the seventh day of the year.
W	Week of month (1-5) where week 1 starts on the first day of the month and ends on the seventh.
Y,YYY	Year with comma in this position.
YEAR SYEAR	Year, spelled out. "S" prefixes BC dates with "-".
YYYY SYYYY	4-digit year. "S" prefixes BC dates with "-".
YYY YY Y	Last 3, 2, or 1 digit(s) of year.