\definefontfamily
  [helvetica]
  [ss]
  [Helvetica]
  [tf=style:Regular,
   bf=style:Bold,
   it=style:Oblique,
   bi=style:Bold Oblique]

\starttext
{\rm The quick brown fox jumped over the lazy dog {\it Optima}}
\setupbodyfont[helvetica]
{\rm The quick brown fox jumped over the lazy dog {\it Optima}}
\stoptext

The first 'Optima' in Latin Modern is Italic, the second one is not. Probably still a font cache issue.