You should do the next steps:
cd ..../bugzilla-4.4/template/en/custom/bug/create/
then you have to edit this file:
vi create.html.tmpl
inside the file you have to find this peace of code:
<tr>
[% INCLUDE bug/field.html.tmpl
bug = default, field = bug_fields.op_sys, editable = 1,
value = default.op_sys %]
</tr>
replace it with this code:
<tr>
[% IF product.name == "ProductA" %]
[% INCLUDE bug/field.html.tmpl
bug = default, field = bug_fields.op_sys, editable = 1,
value = "Android" %]
[% ELSIF product.name == "ProductB" %]
[% INCLUDE bug/field.html.tmpl
bug = default, field = bug_fields.op_sys, editable = 1,
value = "iOS" %]
[% ELSE %]
[% INCLUDE bug/field.html.tmpl
bug = default, field = bug_fields.op_sys, editable = 1,
value = default.op_sys %]
[% END %]
</tr>