Ovo je definitivno naš vez.

Because ovo here is a standalone demonstrative pronoun meaning this or this thing.

• ovo je ... = this is ...
• ovaj vez = this bond / this embroidery / this tie directly before the noun

So in Ovo je definitivno naš vez, ovo does not have to match vez in gender the way ovaj would if it directly modified the noun.

Je is the 3rd person singular present of biti (to be), so it means is.

So the basic structure is:

• Ovo = this
• je = is
• definitivno = definitely
• naš vez = our vez

In Croatian, je is a clitic, and clitics usually go in second position in the clause.

That is why Ovo je ... sounds natural.

You will often see this pattern with forms of biti and short pronouns:

• Ovo je dobro.
• On je ovdje.
• To mi se sviđa.

So the placement of je is following a normal Croatian word-order rule.

Because naš must agree with vez.

Vez is masculine singular, so the possessive form is:

• naš for masculine singular
• naša for feminine singular
• naše for neuter singular

Examples:

• naš vez
• naša kuća
• naše selo

The possessive agrees with the thing possessed, not with the person who owns it.

It is in the nominative singular.

In a sentence like X is Y, the noun after je is often in the nominative when it identifies or names what something is.

So:

• Ovo = nominative
• je
• naš vez = nominative phrase

This is a very common pattern in Croatian identification sentences.

Because Croatian does not have articles like English.

So Croatian simply says:

• Ovo je definitivno naš vez.

English has to choose something like:

• This is definitely our ...

Whether English would use a or the in another sentence depends on context, but Croatian usually leaves that unstated.

Definitivno is an adverb meaning definitely.

It modifies the whole statement and adds certainty:

• Ovo je naš vez. = This is our ...
• Ovo je definitivno naš vez. = This is definitely our ...

It is very similar in use to English definitely.

Yes. Croatian word order is more flexible than English, although some orders sound more neutral than others.

Ovo je definitivno naš vez is a normal, neutral order.

You may also hear variations such as:

• Definitivno je ovo naš vez.
• Ovo je naš vez, definitivno.

These can shift the emphasis slightly, but the core meaning stays similar.

Often yes, depending on context.

• ovo usually feels more like this here
• to can mean that, but in many identification sentences it is also used where English would say that or even this/that is

So both can be possible, but they are not always identical in nuance.
If you are pointing to something immediately present, ovo often feels especially natural.

A rough English-friendly pronunciation is:

OH-vo yeh deh-fee-nee-TEEV-noh nash vez

A few helpful points:

• j is pronounced like English y
• š is pronounced like sh
• v is like English v
• z is like English z

So:

• ovo ≈ OH-vo
• je ≈ yeh
• naš ≈ nash

Yes. It is a normal and common Croatian adverb.

It is clearly related to international words like definite or definitive, so it may look familiar to an English speaker. In everyday Croatian, definitivno is widely used and sounds completely natural.