This script takes an OPML file exported from OmniOutliner and turns it into a markdown file with the specified depth of headings. Use the share sheet in OmniOutliner to copy the OPML file to Drafts, then run the action.
Lines with a depth greater than the maximum heading depth are converted to markdown lists with the appropriate level of indentation.
FYI, Omni Automation, which is the dual-platform (iOS/macOS) JavaScript automation implementation in OmniGraffle and OmniOutliner, can be used to quickly transfer data from those apps into Drafts.