Go into the ODS mods folder, open OPTIONS.INI. In the graphics section you will find a CloudDetail parameter. As far as I know that is not adjustable from any game options screen. Try playing with that parameter and see if it helps.
What OS are you running? SF2 with DX10 support blows the doors off DX9 but that really requires you to be running Win 7 and have a DX10 capable card. I've heard there's a DX10 for WinXP but have not tried it.
The other issue with ODS (which is a good news/bad news thing) is that the missions seem to be much busier in terms of number of flights, etc. than the stock SF2 missions. Managing a lot of missions takes processing power and that may be hitting you as well. That's not as simple a fix in that you need to modify the campaign to reduce the number of flights for a given mission. There are some posts around that describe what to do. I think the various "supply rate" parms affect somewhat how many missions are generated.