An ornament is a pattern of plant for example. A decoration is some stars, baubles, bows etc. It's all that makes our house looks better/ more beautiful. :)

If one wished to do a play on words with two countable nouns, then one could have such an example: "A diamond brooch is an ornament but not a decoration. A military medal is a decoration but not an ornament."

For the general learner, let's confine this explanation to the use of these two words as uncountable nouns. If so, then "ornament" is a subset of "decoration". Example: The interior decoration of the college dining hall was much enhanced by heraldic ornament.