DiOS is a model operating system designed for use in reproducible testing and verification of application-level software. Its main mode of operation is as a self-contained, POSIX-compatible system with 100% deterministic behaviour, fully isolated from outside environment (and hence interference). The goal of this thesis is to design and implement an extension of DiOS which would make it possible to record and then replay traces of executions that were allowed to interact with a real (host) operating system. The motivation for this mode is capturing interactions that are too complex or too costly to model explicitly. To capture traces, DiOS needs to offer a mode in which it forwards system calls to the host operating system, with a certain degree of cooperation from its immediate environment (usually the virtual machine it is executing in). The traces should be captured in a transparent, and to the extent possible, human-readable format. The interface between the guest (DiOS) and host operating systems should be as generic and simple as possible. The second mode of operation to be implemented is the replay itself, in which DiOS loads a previously recorded trace and when the program executes system calls, they are matched against the trace (based on their input arguments) and their recorded effects (return value and output arguments) are replicated.