The Original Form of Okonomiyaki

Okonomiyaki was originally a simple cracker, called sembin. It was made with flour dissolved by water. It looked like a crape and people ate it rolling some side dishes. 

The First Okonomiyaki in Japan

In Muromachi period, Sen-no-rikyu use "Funoyaki" as the snack in tea time. The dough which is made from udon flour, water and sake was stretched thin, grilled, and then it was rolled with miso. It's said as the original Japanese okonomiyaki. 

The Beginning of Okonomiyaki

Before the war "monja-yaki" and "dondonyaki" became popular among children. They eat "Dondonyaki" with worcester sauce and it is known as "一銭洋食/Western food in one coin" because they thought putting the sauce on foods is Western things.
After the war, people struggled with starvation. At that time, people realized 一銭洋食 is a good and cheap meal to satisfy their appetite. That's why they started to remake this food not just for kids but adults.
The chefs put a little of pork on it, changed the name from 一銭洋食 to お好み焼き/okonomiyaki, and made it open for adults too. 

Okonomiyaki became popular

After 1955 or so, people began to use various kinds of ingredients from place to place. They came to regard okonomiyaki as not just the food to make the stomach full but as the great food for adults.

References

お好み焼きの歴史 | お好み焼きの歴史と未来 | オタフクソース (otafuku.co.jp)