Learn Finnish: Sen uuden teoksen loi nuori nainen. - That new work was created by a young woman.

No, the Finnish sentence is strictly in the active voice! The literal word order is "That new work (object) created a young woman (subject)." Because Finnish uses word endings to show who does what, you can safely put the object first to emphasize it (topicalization). In English, we usually have to switch to the passive voice to keep the focus on "that new work" without the sentence sounding nonsensical.

Since the word order is Object-Verb-Subject, how do we grammatically prove who is creating what?

We rely entirely on the case endings. Nuori nainen (young woman) is in the basic nominative case with no extra endings, which marks it as the subject doing the action. Sen uuden teoksen (that new work) has the -n ending on every word, marking it as the total object receiving the action.

What is the base form of the verb loi, and how is it formed?

The infinitive is luoda (to create). To form the past tense, the final -a drops off to make room for the past tense marker -i. In verbs that have a double vowel like uo before the ending, the first vowel drops out when the -i is added. So, luoda becomes loi (he/she/it created).

Why does the word teos look so different as teoksen?