This is OmniOutliner. Of course you can make it work :-)
One straightforward change would be to simply write your codes as Innn.nn which will allow the text sort to work:
Or do it with just the numbers, but padded with leading zeros to the same width:
Yet another option would be to add another column (the sort key) which will mirror the ICD-10 codes, except as a number, not text. Then you can sort on that sort key column and have your document be ordered the way you want. Or, if you wanted to elide the ICD-10 code from the outline proper for some reason, you could make the sort key column be the actual text code, written with the same padding/field width as above. Using a separate column for the sort key will allow you the greatest flexibility in what text you put in the outline.
