The most immediate way to watch the film with English subtitles is through community-shared versions on video platforms:
: Full versions of the film with English subtitles integrated have been uploaded to YouTube. Search for terms like "Ostia 1970 full movie English subtitles". Ostia subtitles English
If you already have a digital copy of the film and need a separate subtitle file (typically in .srt format), you can search these popular databases: The most immediate way to watch the film
: A massive database often recommended for hard-to-find foreign films. Ostia subtitles English