On je žrtva oluje.

Because žrtva is a grammatically feminine noun, but it can refer to any person, male or female.

So:

• On je žrtva oluje. = He is a victim of the storm.
• Ona je žrtva oluje. = She is a victim of the storm.

This is a common thing in Croatian: grammatical gender and real-life sex/gender do not always match.

Oluje is in the genitive singular.

The basic dictionary form is:

• oluja = storm

But after žrtva in the sense victim of something, Croatian normally uses the genitive:

• žrtva oluje = victim of the storm
• žrtva nesreće = victim of the accident
• žrtva rata = victim of war

So oluja changes to oluje because it is not the subject here; it is part of the expression žrtva + genitive.

Because Croatian often expresses of simply by using the genitive case, without a separate word.

So:

• žrtva oluje literally looks like victim storm-GEN
• but in natural English it means victim of the storm

This is very common in Croatian. English uses prepositions more often; Croatian often uses case endings instead.

Je is the 3rd person singular present tense of biti = to be.

So:

• on = he
• je = is
• žrtva oluje = a victim of the storm

Together:

• On je žrtva oluje. = He is a victim of the storm.

Sometimes yes, if the subject is already clear from context.

Croatian often drops subject pronouns, but with je there is a special issue: je can mean is for he, she, or it, so the pronoun may be useful when you want to make the subject explicit.

Compare:

• On je žrtva oluje. = He is a victim of the storm.
• Ona je žrtva oluje. = She is a victim of the storm.

If you just say:

• Je žrtva oluje.

that is not a normal neutral full sentence by itself.

You could omit the pronoun in some contexts with a different word order, but for a straightforward learner sentence, On je žrtva oluje. is the clear standard version.

The given order is the most neutral and natural:

• On je žrtva oluje.

Croatian word order is fairly flexible, but changing it changes the focus or emphasis.

For example:

• Žrtva oluje je on. = It is he who is the victim of the storm.
• On je olujina žrtva. = another possible phrasing, more literary or stylistically marked

So yes, you can change the order, but the original sentence is the best neutral choice.

Because Croatian does not normally use od here.

With žrtva in the meaning victim of, Croatian normally uses a bare genitive:

• žrtva oluje
• žrtva rata
• žrtva prijevare

Using od would sound unnatural or incorrect in this pattern.

So the correct structure is:

• žrtva + genitive

not

• žrtva od + noun

It can mean both, depending on context.

Common meanings of žrtva include:

• victim
• sacrifice

In On je žrtva oluje, the meaning is clearly victim, because a person is affected by a storm.

So the context tells you which meaning is intended.

Because Croatian has no articles like a, an, or the.

So:

• žrtva can mean a victim or the victim
• oluje can mean of a storm or of the storm

The exact meaning comes from context.

In many learner translations, On je žrtva oluje is rendered as:

• He is a victim of the storm or
• He is the victim of the storm

depending on the situation.

A rough English-style guide is:

• On ≈ ohn
• je ≈ yeh
• žrtva ≈ zhrt-va
• oluje ≈ oh-loo-yeh

A few important points:

• j is pronounced like English y
• ž sounds like the s in measure
• r in žrtva is a trilled or tapped r

So the whole sentence is approximately:

ohn yeh zhrt-vah oh-loo-yeh

Here are the basic forms:

• on = he
• biti = to be → je = is
• žrtva = victim
• oluja = storm

In the sentence:

• on stays on
• je is the present tense form of biti
• žrtva stays in its basic nominative form because it is the predicate noun
• oluja changes to oluje because it is in the genitive singular

No. The subject is on.

The structure is:

• On = subject
• je = verb
• žrtva oluje = predicate noun phrase

So in grammatical terms, žrtva is not the subject; it is what he is.

That is why the sentence means:

• He is a victim of the storm

not

• The victim of the storm is he
  unless you deliberately change the word order for emphasis.