Oracle high CPU usage even when not in use

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We have a couple of servers here which have been setup with the same version of Oracle but are used for 2 different services. One of which is ok but the other seems to be having higher than expected CPU usage. It does spike when it is being used, but my actual concern is that when it isn't being used it never drops below 50% even at night/weekends. It's not one of my servers (and I know virtually nothing about Oracle) and the person who set it up is off at the moment so has anyone got an idea of why it might be using so much CPU? From what I can see there is nothing else running which is causing the problem so it just looks like Oracle is using all the CPU but surely it should drop off when not in use?

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Oracle will eat anything you throw at it, but this does seem odd. Is it dedicated for Oracle or is it using oracle for another service?

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It is a dedicated Oracle Server. It turns out the person is due to be in today(so I've been told) so hopefully they may be able to look at it. It just seemed really odd as it looks like it has just taken 50% CPU in reserve but it doesn't let it go when not is use. I assumed it is a Oracle install/config issue but didn't really know where to start looking.