You may want to add a field to your WordPress posts using the Advanced Custom Fields plugin. But you may also want that field's value to fall back to a default value, if for whatever reason, the ACF field is not provided by the user.
Initially this seems easy to do in Advanced Customer Fields, as they do provide an opportunity for you to specify a default value.
The problem is that any default value supplied in the way shown above, will not apply to any existing posts. If this is a problem for you, you can address the default value in another way by adding a quick code block and changing the data source of the field in your Oxygen Template.
Here is an example where we're designing an Oxygen template and want to specify a different hero image for each post.
The hero image URL for each individual post will be supplied via an advanced custom field. But unfortunately, in our case our website already has a large number of posts. To avoid having to re-visit each post and re-save it, we'll have to come up with a faster and easier way to specify a default for any existing posts that don't have a specific hero image supplied.
Please note, if you are using this method you do not need to specify a default value within ACF. You can - but as already pointed out it will not apply to existing posts. But it will also not be over-written on new posts by the coded workaround either. So if you were to make the ACF default different to the code block default, you'd end up with different defaults for missing posts. Just thought I'd point that unlikely scenario out, LOL.
That's it. When you specify a specific URL through ACF on your post, you'll get the image of your choice, and when you don't you'll get the default stated in the code.