You can issue LabTalk commands from Origin C using the LT_execute() method. Also LabTalk objects such as NLSF, FFT etc can be accessed from OC. Existing OGS files and sections can be used as well by executing a call to such sections.
For more details, please see the Programming help file and the topic: Programming Guide->LabTalk Access (the above is in 7.5 help files - should have similar topic in 7.0)
If you have upgraded to Origin 7 or 7.5 from an earlier version most of your old LabTalk scripts should still work. It's not necessary to rewrite them as Origin C functions.