Drain the excess oil off the cans of tuna and add to your mixing bowl.
Dice the onion into approximately quarter inch size pieces. Add to the mixing bowl.
Cut the top and bottom off the radishes and slice into thin rounds. Add to the mixing bowl.
Add the fresh or dried oregano, Salt, pepper, Olive oil, vinegar, and lemon juice.
Mix all of the ingredients gently. You want to distribute the ingredients evenly; however you do not want to overmix the tuna and make it mushy. There still should be individual flakes of fish recognizable in the finished dish.