If you do not know the sequence but know the length of the RNA, you can use the average molecular weight of the 4 bases (or bp's), and multiply it by the length (no. of bases or bp's) of your RNA. This will give you the total MW of your RNA.  If you multiply the concentration in ug/ul by the volume you are using, this will give you the no. of ug's.  Knowing the total MW of your RNA , you can convert the no. ug's to no. of nanomoles of your RNA. You do not need the OD, except for relating it with the concentration in ug/ul. 

Thank you very much Dr. Vicente! It is helpful. I didn't get the sequence and also the basepair length, though it helped me to calculate the ug value assuming the standard length of RNAi (21bp in general) and subsequently calculated the nanomoles.

No, you cannot calculate molar amount without knowing the nature of molecule (sequence in this case) of anything.... that's just the way chemistry tends to work.  ;)