Can Oscar Koi Carp be Tank Mates?

Answer 1

Not very effectively since Koi are coldwater fish and Oscars are tropical.

As above answer... It is potentially problematic, but can be done. Both types of fish are major producers of waste. Ammonia and nitrite cycles have to be kept in balance, and then, there is the question of "room". Koi will be large. Eventually, they will need a pond. Oscars can grow as much as an inch a month when young, if fed well. The water temperatures overlap enough to allow for them to room together by approximately 5 degrees...another something to watch. In other words, it will be a lot of work. I actually have them as tank buddies right now, but as I do this, plans are already in the works for future living arrangements. While small, Oscars and Koi make a lovely splash of color in the tank together, and they do get along if close in size (depending on the individual personality of your fish), just know what you plan to do with them when they grow. The Oscar will probably end up by himself later just for his sheer size, and the Koi will have to have more space as well.