it's 11gR2 ---18.104.22.168
PID USERNAME LWP PRI NICE SIZE RES STATE TIME CPU COMMAND
18583 oracle 46 59 0 27250M 26740M sleep 74.8H 0.27% oraagent.bin
According to the below notes:
ORAAGENT or ORAROOTAGENT High Resource Usage [ID 1062676.1]
Bug 10168006 - oraagent process memory growth periodically [ID 10168006.8]
confirms it is a known bug and as a work around just the kill the process and oracle will re spawn the process.