Double diacriticaled letter

I’m an non-english speaker absolute newbie to latex and this forum (so many reasons to despair !). I’m trying to use it to write ancient greek.I’ve managed to make single-diacritical letters alright.
But I would like to stack a first diacrital sign for the accent (ó for example, made with '{o}) with another diacrital for letter quantity (ō or ŏ, made with ={o} or u{o} ).
With :

documentclass{article} 
usepackage[polutonikogreek,frenchb]{babel} 
newcommand*{tg}[1]{textgreek{#1}}  
begin{document} 
tg{={'o}}
end{document}

I’m getting

Which would be perfect, but I would like the acute diacritical on top.

With :

documentclass{article} 
usepackage[polutonikogreek,frenchb]{babel} 
newcommand*{tg}[1]{textgreek{#1}} 
begin{document} 
tg{'{=o}} 
end{document}

I’m getting

with the acute diacritical not on top, but on the left. :/
Any idea to solve this issue ?
Many thanks, o wise lateX wizards !

documentclass{article} 
usepackage[polutonikogreek,frenchb]{babel} 
usepackage{teubner}
newcommand*{tg}[1]{textgreek{#1}}  
begin{document} 
tg{aAm{a}Gm{a}Cm{a}oAm{o}Gm{o}Cm{o}}
end{document}