I did the span, but it only added below it, not beside or aligned with the field.

What do you mean, comment the span? That will not read the line, it will only comment.

mention the code like this
<label for="odometer"/>
<div class="o_row">
<field name="odometer" class="w-25"/>
<span>your text</span>
</div>

you have create the label for the field and create the div tag in side the div mention your field and the text it will come in the same line.

You have mention the span tag inside the field tag. that's wrong it will not get the text on right side need to create the label for the field and create the div tag. inside of the div tag you need to mention the field and the span tag.

Thank you very much!