Siebel Application does not support REST web services on Siebel 8.0.x version, inbound .
REST inbound web service support is GA in release 126.96.36.199.
Specifically for the OUTBOUND scenario, nothing new is added by release 188.8.131.52.
Outbound scenarios are similar to "pure" HTTP calls and EAI HTTP Transport business service is already capable of doing that on any siebel release.
Siebel does have the EAI HTTP Transport, XML converters , eScript programming language and workflows, all of which can be leveraged to build a custom solution to call OUTBOUND REST services.
Customization is needed to build the message to be sent out and to read the message received.
EAI HTTP Transpor can make an http call to send data and receive responses back.
This works in all Siebel versions.