A: LAST_DAY does not only return the last day of the current month; it returns the last day of the month based on the date argument passed to it, which may not necessarily be the current month. Thus, statement A is incorrect.

B: CEIL requires a numeric argument and returns the smallest integer greater than or equal to that number. Thus, statement B is incorrect.

C: ADD_MONTHS function adds a specified number of calendar months to a date. This statement is correct as per the Oracle documentation.

D: ADD_MONTHS can work with a character string if the string can be implicitly converted to a DATE, according to Oracle SQL data type conversion rules. Therefore, statement D is correct.

E: LAST_DAY does not specifically return the last day of the previous month; it returns the last day of the month for any given date. Thus, statement E is incorrect.

F: CEIL returns the smallest integer greater than or equal to the specified number, not the largest integer less than or equal to it. Hence, statement F is incorrect.

G: LAST_DAY returns the last day of the month for the date argument passed to the function, which aligns with the definition in Oracle's SQL reference. Therefore, statement G is correct.