OMB is just the payer or form #.  You mean RMD Required Minimum Distribution.  Say all of it is the RMD.  If you are 70 1/2 it will ask you if it is the RMD.  Say yes.  Anything your pension pays you is considered to be the  RMD.  Traditional pensions automatically fulfill the rules of an RMD.

I looked it up.  OMB stands for Office of Management and Budget.  They produce the IRS blank forms. Most tax forms start with a OMB No. in the upper right.