Here is the assortment of sauces I could find in New Zealand. From left to right: BBQ sauce, ketchup, soy sauce, and oyster sauce.
Mix together using the proportions listed.
This way, you can spread it generously over your okonomiyaki.
Story Behind this Recipe
This is an improved variation of my flat mate's recipe. Thanks! Japanese ingredients are expensive when living abroad, so this kind of knowledge is helpful for you. These ingredients used are available anywhere, so you can use this okonomiyaki sauce as much as you like, without being sparing.
If you are okay with the smell of the oyster sauce, omit the BBQ sauce and use a 1:1:1 ratio of the other ingredients. I prefer a ratio of 2:1:1:1. The oyster sauce is optional. The others are the basic components of okonomiyaki sauce.