There are two OpenType flavors, TrueType based and PostScript based OpenType. For more background information about this see: What is the difference between TrueType and OpenType?
FontCreator supports one of them. The one based on TrueType outlines. Usually such font files have a .ttf extension, but a .otf extension is valid as well.
If you open a PostScript based OpenType font (prefered extension is .otf), the outlines (CFF) will be converted to TrueType (glyf) outlines, and among that the instructions for hinting are lost.
You can safely open and save a TrueType based OpenType font, but there are several restrictions if the font contains unsupported tables.
First make sure the “Remove” flags are unset, especially “Remove unsupported tables” before opening the font. You can change these flags through the Options window. To see or change these settings, go to the main menu, and open the Tools menu, click Options, and then click the Font tab.
Then you can edit such font, but you will most likely break something within the unsupported tables if you add, remove, or change the order of the glyphs.