You can try adjusting the top & bottom margins of the P element inside each of the Markdown-produced OL > LI items. Make a new tiddler and tag it with $:/tags/Stylesheet. In the tiddler body add some CSS for adjusting those paragraph margins, like this...
ol > li > p { margin: 0.25em auto; }
This will have equal top & bottom margins, adjust the first margin value to adjust the spacing.
If you want to adjust them each independently, try this instead...
ol > li > p { margin: 0.25em auto 0.25em; }
Adjust the first value for the top margin, third value for the bottom margin.