How to cast date in derived column using format mask?

My customer has asked me to define a column in the RPD that uses to_char and a format mask to display the date in a MMYY format.  I found the following post in this forum from 2013 which has not been answered:  https://community.oracle.com/thread/3391431?q=cast%20date%20formatmask

So, I decided it may be best to repost in hopes of getting a second look.  I'll be happy to update the other post if the answer is found here. 

Here is the derived column that they want defined:

decode(cog, 'AA', to_char(test_date, 'MMYY'), decode(code, 'C', to_char(run_date, 'MMYY'), decode (code, 'E', to_char(run_date, 'MMYY'), to_char(expire_date, 'MMYY'))))