(Not a teacher, nor bhaisahab)
If your teacher wants you to 'prove' the existence of the word 'joyous', you should have given him/her a dictionary. It is likely to have it's own entry. If that fails, you can use the following:
BNC Simple Search
There is a list of examples from the British corpus of 'joyous'. Also if you would like to find examples from the American corpus, go to this website:
Corpus of Contemporary American English (COCA)
Click 'enter' then search for 'joyous' in the search box at the left. It comes up with hundreds of examples that you can use.
Regarding the adjective of 'oppress' - 'oppressed' and 'oppressable' are adjectives from this word, but meaning different things. 'oppressed' is someone who is oppressed. 'oppressable' is someone who is able to be oppressed.
Student or Learner