\input luaotfload.sty

\font\testa={LM Roman Slanted 10 Regular} at 10pt
\font\testb={LM Roman 9 Italic}           at 10pt
\font\testc={TeX Gyre Termes Bold}        at 25pt
% Also testing with absolute filename, please change the path according to your 
% system
\font\testd=file:/usr/share/texmf/fonts/opentype/public/lm/lmmono10-italic.otf

\testa abcd ABCD\par
\testb abcd ABCD\par
\testc abcd ABCD\par
\testd abcd ABCD\par

\bye