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You could capture an image of the OEM partition, delete it, extend your C drive by the desired amount MINUS the amount necessary to bring back the OEM partition, then restore the OEM partition into that space. I personally consider that safer than live repartitioning tools, and it would almost certainly be faster. And if you already back that up, you can just restore that partition from your existing backup rather than backing it up again.
Or as Froggie said, back up your system, then immediately run a restore that includes a partition layout adjustment.
Or you could just dump the OEM partition entirely.