I looked in FamilySearch and you have quite good resources to find. I would suggest to not start with the census, but with the cementry register, as you know around which date your great grandmother died.
The list of resources in FamilySearch is this one: https://www.familysearch.org/search/catalog/results?count=100&placeId=1115843&query=%2Bplace%3A%22Spain%2C%20Andaluc%C3%ADa%2C%20Sevilla%2C%20Osuna%22&subjectsOpen=1115844-50,1445905-50,1531141-50
The cementry is this one: https://www.familysearch.org/search/catalog/2821194
Concerning census/registry of inhabitants, I would go as second step.
Unfourtunately, women's suffrage was fully approved in Spain in 1931. That means that any census in Spain before that date will not contain your great grandmother. However, depending on the city, the register of inhabitants (called "padrón" in spanish) might contain women. However, the brothers of your great grandmother will be present. Just wanted to mention that in terms of searching strategy.
When addressing the census, also take into account the voting age (over 18 years), so you should look in census over 1901 for your great granfather. Your great grandmother was 18 around 1908, but you do not have any idea about her brothers... so I would look around 1910-1920.
I looked in FamilySearch and I found that actually several census and reigsters of inhabitants (padrón) here: https://www.familysearch.org/search/catalog/2821195?availability=Family%20History%20Library
I also double checked and they are accessible from internet (you can see the images) but they are not digitalized (so it will be a real search around several hundred of pages...)
I checked the "padrón" and I found that women and men were included but it looks that the minimum age for being included was around 14 years old. That's making your starting time around 1897.
Be aware as well that, for some reason, sometimes the age in the census was not accurate.
Based on that, I would suggest the following:
Grandfather: use the census from 1906 to 1916, starting in the latest year. Once you locate the street in the first one it will be easy for you to locate the person in the census (you go straight away to the street). You might need to find 2 streets, when he was living wiht his parents (your great-great parents) and when he was living wiht your great granmother (btw, both locations might be the same...). With that street you can go to "padrón" or "census" and find everybody living there. I do recommend starting wiht census because there will be half of the people to look through.
Grandmother: start with her as second step, once you know the places your granfather was living. Depending on your result above you might be able to use "padrón" to find her or you might need to look for the brothers of your grandmother. Problem is that she was 26 in the last census, and she might not have married by that time (so grandfather might not help wiht grandmother, that's why grandfather search first is the best strategy).
Another research idea is to contact the cementry of Osuna, they might have a registry of death and might find your parents (with the exact death date you will be able to request a certificate and later on finding the right location)
Hope this helps... if I think in any other strategy I will try to include here.