We departed OSH IFR from Runway 27 on Wednesday 22 July. We were cleared to 3000' and directed to contact Milwaukee Departure. Just as we were passed to Milwaukee departure and passing 1800' guess what! Two aircraft at the same altitude, one a piper dead ahead, and a second slightly to our right on the FISK arrival. A brisk turn to the left maintained our separation at about 300 from the second aircraft, a single engine Piper. Neither aircraft appeared to see or take note of us. (Note: no flames please - I understand that IFR aircraft in VMC are responsible for visual separation, which we accomplished.) The purpose of this post is to point out the possible conflict between VFR aircraft under positive control of the FISK arrival controllers with IFR aircraft under positive control of OSH Tower/Milwaukee Departure. The FISK and Milwaukee Departure controllers are not talking to each other. A NASA report has been filed recommending that IFR aircraft maintain 1300' until clear of the FISK arrival.