Oregano substitute suggestions

Italian seasonings has oregano so I would skip that. How about some thyme, parsley, basil, and/or chives.

The most obvious substitute would be marjoram which is the same genus.

Personally I would consider ditching the dried herbs since my garden is bursting with fresh ones (thyme, tarragon, basil, chives, parsley, oregano, marjoram, sage) right now.

This dish is flexible enough where an herb substitution isn't going to cause failure. It's a salad seasoning, not something that contributes any sort of structural function like flour in bread dough.

Ultimately it's your call since the goal is for you to put something on the table that your dinner guests will enjoy not the favorite herbs of random people from the Internet.