Creating new single line classes in ODT output from mk4ht oolatex

There are several issues with your configuration. Fist of all, in testpkg.4ht:

renewcommandSectionTitleTest{a:SectionTitleTesttmp:SectionTitleTestb:SectionTitleTest} 

The original SectionTitleTest takes one argument, but you redefine it as argument free. Because of this, all content of the configuration is inserted before the text you pass as argument to SectionTitleTest. The correct redefinition would look like this:

% provide configure for new command. we can choose any name
% but most convenient is to name hooks after redefined command
% we declare two hooks, to be inserted before and after the command

NewConfigure{SectionTitleTest}{2}

% now we need to redefine SectionTitleTest. save it to tmp command
% note that `:` can be part of command name in `.4ht` files.
lettmp:SectionTitleTestSectionTitleTest

% now insert the hooks. they are named as a:hook, b:hook, ..., h:hook
% depending on how many hooks were declared

renewcommandSectionTitleTest[1]{a:SectionTitleTesttmp:SectionTitleTest{#1}b:SectionTitleTest}

The second issue is that you use HTML tags. In the ODT output, you must use OpenDocument tags. Also, ODT doesn't support CSS, it uses special XML instead.

The updated .cfg file:

Preamble{xhtml}

% to simplify things
defmyendpar{ifvmodeIgnoreParfiEndP}

Configure{SectionTitleTest}{myendparHCode{<text:p text:style-name="section-title">}}{HCode{</text:p>}}
NewConfigureOO{section-title}
ConfigureOO{section-title}{<style:style style:name="section-title" style:family="paragraph" style:class="text">
    <style:text-properties style:text-underline-style="solid"
                           style:text-underline-width="auto"
                           style:text-underline-color="font-color"
    />
</style:style>}

begin{document}

EndPreamble

The <text:p> element is used for paragraphs. It needs to use the text:style-name attribute to select the paragraph style.

The paragraph style is defined using NewConfigureOO and configured using ConfigureOO. I've used the same name as in text:style-name, but it is not necessary. The correct style name must be used <style:style style:name="section-title"> though.

The provided style configures the section title to produce underlined text. Use <style:text-properties> attributes to change the formatting according to your needs.

This is the result in LO: