Hello Marko, and welcome to the Community.
Yes it is possible but only for some categories of jobs. This is an extract from a help article:
"If the portfolio item is in the Design & Multimedia, Software Development, Web Development, or Writing & Translation categories, you'll have the opportunity to link it to an oDesk contract."
Details on how to link a portfolio item and a contract can be found here towards the end
https://support.odesk.com/entries/23103037
Remember the guidelines to linking your client's copyrighted material though.
---- easy like Sunday morning ----