The Object Constraint Language (OCL) is a formal, declarative, and side-effect free language, standardized by the Object Management Group, for specifying constraints or queries on models specified in the Unified Modeling Language (UML). OCL was designed with the aim to bridge the gap between natural language and traditional formal languages requiring a strong mathematical background to understand and apply. OCL, along with UML, have been applied in practice for various purposes such as facilitating automated model-based testing. In most of such contexts of OCL, engineers with software engineering backgrounds specify OCL constraints. However, it is still a challenge for constraint authors (e.g., medical coders) who have no such background to apply OCL for other purposes (e.g., specifying medical rules). In this direction, in our previous work, we proposed a user-interactive specification framework, named iOCL, for facilitating OCL constraint specification and validation. The aim was to ease its adoption in practice in a wider application scope. In this paper, we present a pilot experiment that was conducted to assess the practical applicability of iOCL in Cancer Registry of Norway with real users of iOCL in terms of specifying medical cancer coding rules with iOCL. Results of the pilot experiment showed that, with iOCL, time to specify OCL constraints can be significantly reduced as compared to directly specifying OCL constraints without the tool support. In addition, participants of the experiment found that iOCL is easy to use.