When saving the font, or afterwards, you can rename the extension from *.ttf to *.otf.
I have no idea if this is enough for the Mac to recognise the font as OpenType, but Windows is quite happy to use such fonts without digital signing. Adding digital signing to give the font an OpenType icon is an expensive proposition.
How to get an OpenType icon on a OpenType TTF Flavored font file?
When adding OpenType Features with the OpenType Compiler as described in this tutorial, the *.ttf font is renamed with an *.otf extension. My free fonts use this method.