Although you can not change the resolution you can however SERIOUSLY improve the graphics.... An all as easy as 123.....
ENB Series, which is a mod made by sone russian guy, changes the way DirectX works. By simply placing two files in \Omsi\
You will boost graphics unbelievably.
I didn't have time to make the screen shots for you and the playglobe.net forum is down at the moment and they have all the screen shots.
In this file you will find:
d9dx.dll (Place in \Omsi\)
and you will find about 8 .ini files.
Choose the graphics setting you want (each file name describes what in does)
Then place that .ini file in the \Omsi\ root and make sure it is named "enbseries.ini"
Hope you enjoyHeres the download link