That is not the way to set up the configuration bits. Look at the C30 User Guide, section 6.6.1.
Also read Section 8 of the data sheet and the Family Reference Manual Section 6 which explain the oscillator setup.
What is frequency of the crystal you are using? I assume it is 8MHz since you are measuring RC15 and this pin is acting as the OSCO line to drive the crystal. (It is NOT Fosc/2 in this configuration.)
If that is correct, then you actually need to use XT (and XTPLL) mode - look at Section 6.5 of the FRM to see the correct settings based on the crystal frequency.
I suggest that you have a look at some of the example code that is available from Microchip and also through a Google search to see how others have set up the oscillator. While the data sheets and FRM sections actually will tell you just about everything you will need to know to use these MCUs, there is a fairly steep learning curve involved, part of which is learning how to read these documents.
Susan