To execute MPI applications reliably, fault tolerance mechanisms are needed. Message logging is a well known solution to provide fault tolerance for MPI applications. It as been proved that it can tolerate higher failure rate than coordinated checkpointing. However pessimistic and causal message logging can induce high overhead on failure free execution. In this paper, we present O2P, a new optimistic message logging protocol, based on active optimistic message logging. Contrary to existing optimistic message logging protocols that saves dependency information on reliable storage periodically, O2P logs dependency information as soon as possible to reduce the amount of data piggybacked on application messages. Thus it reduces the overhead of the protocol on failure free execution, making it more scalable and simplifying recovery. O2P is implemented as a module of the Open MPI library. Experiments show that active message logging is promising to improve scalability and performance of optimistic message logging.