Ruzzo-Simon-Tompa oracle access mechanism
A definition of oracle access that works for small circuit complexity classes (the $AC^k$ and $NC^k$ hierarchies) as well as for logarithmic space classes, with the property that all known inclusions relativize, can be found in this paper: 

Klaus Aehlig, Stephen Cook and Phuong Nguyen: Relativizing Small Complexity Classes and their Theories, CSL 2007, Springer LNCS 4646