Opportunistic network (Oppnet) is a class of networks where connections between the nodes are not permanent. The nodes are continuously moving and some nodes even switch off their batteries to conserve energy. Reliable delivery of messages in Opportunistic network is one major inherent issue. It is unreliable in the sense that once the source node has forwarded its message, then it will never get to know about its status in the network like whether the message has got discarded at an intermediate node or at the destination node (due to buffer overflow) or the successful delivery of the message has taken place. This work tries to make Oppnet as much reliable as possible. It proposes a reliability protocol named as “Reliability in Oppnet” (RIO). RIO improves the routing in Oppnet and works in parallel with the existing routing protocols. It makes the source node aware about the status of message so that if an error occurs then the source node can take suitable action to resend the message. It considers the redirection error, buffer overflow error, Time Limit Exceeded (TLE), parameter problem and destination unreachable errors that may occur inside the network. RIO has been tested using ONE simulator and implemented with Spray and Wait routing protocol. Results show that the RIO with Spray and Wait protocol outperforms normal Spray and Wait protocol in terms of average message delivery probability.