Here is the answer to your question, at least what I understood you wanted. I don't know if this works for Open Office but it is the way it is done in MS Power Point.
Use the slide master to change the font throughout the presentation
If you're using one slide master (slide master: The slide that stores information about the design template applied, including font styles, placeholder sizes and positions, background design, and color schemes.), you can change a font style on the master and the change is reflected throughout the presentation. If you are using multiple slide masters (such as when you have applied more than one design template (design template: A file that contains the styles in a presentation, including the type and size of bullets and fonts; placeholder sizes and positions; background design and fill color schemes; and a slide master and optional title master.) to your presentation), you must change the font style on each master.
On the View menu, point to Master, and then click Slide Master.
In master view, thumbnails of each slide master (in pairs that include a title master (title master: The slide that stores information from the design template pertaining to styles on title slides, including placeholder sizes and positions, background design, and color schemes.) for each) appear to the left of the Slides tab.
If you are using multiple masters, select the slide master thumbnail or title master thumbnail whose font style you want to change.
On the slide, click the title text or the level of body text to which you want to apply a new font style.
(Remember, on the master, you are only changing the font, not the text.)
I hope this is useful to you next time. There is always a next time
Let us know how was your presentation anyways.