Ehm... I am not 100% sure if I understood your problem, the text is a bit confusing. Forgive me if I misunderstood, perhaps you could explain your problem once more.
An index (目錄) is a collection of headings (標題). At least the "standard" index is made this way. That means, you have to format your text correctly, or OO can not help you create the index.
If your text is larger, it will have more headings. Some of those headings are more important, some less. In Chinese, people sometimes speak of 大標題 and 小標題. However, in OO, these are the names for two different styles: Title and Subtitle, which are special, not "regular" headings.
So we have heading 1, heading 2 etc. Heading 1 is for the most important headings, heading 2 for the next level of headings, heading 3 for the next...
OO will use an indent in the index, so you can easily see the "importance" of a heading. The more a heading is set to the right in the index, the "less important" it is.
So, please format your documents that way first. Next, you will find that OO is using paragraph styles also for the index. In the English version, they are called "Content 1, 2..." and "Content Heading". Modify them, if you want different fonts, distances etc.
And of course you can also choose your own index title before you insert the index. So, basically, you can do everything you could need.
If you want to check, I have two files I am using for my classes. They look the same, but one is correctly formatted, the other more "classic":
Correct formattingBad formatting
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