Okonomiyaki sauce is expensive outside Japan, so I whipped up my own version, and it worked! Now that I can make my own, I've been making okonomiyaki more often.
Depending on the brand, the sweetness and saltiness vary, so try starting out with a 1:1:1 ratio. ABC Sauce is a thick and sweet soy-sauce-based sauce available in Indonesia. If unavailable, use oyster sauce.