Ovaj tjedan radim prekovremeno, jer imamo previše posla.

Both are possible in Croatian.

In this sentence, ovaj tjedan is an adverbial expression of time, and it is best understood as the accusative form. With a masculine inanimate noun like tjedan, the accusative singular looks the same as the nominative, so you see ovaj tjedan.

You can also very commonly hear ovog tjedna, which is genitive and means almost the same thing: this week / during this week.

So:

• Ovaj tjedan radim prekovremeno.
• Ovog tjedna radim prekovremeno.

Both are natural.

It is functioning as an expression of time, so here it is effectively in the accusative singular.

The tricky part is that for masculine inanimate nouns, nominative singular and accusative singular are often identical:

• nominative: ovaj tjedan
• accusative: ovaj tjedan

So although it looks like the basic dictionary form, its role in the sentence is that of a time expression: this week.

Because Croatian often leaves out subject pronouns when they are already clear from the verb ending.

Radim already means I work / I am working. The ending -im shows first person singular.

So:

• radim = I work / I am working
• ja radim = I work / I am working, but with extra emphasis on I

You would add ja only if you want contrast or emphasis, for example:

• Ja radim prekovremeno, a on ne radi.
  I’m working overtime, but he isn’t.

Croatian does not have a separate tense like the English present continuous.

The simple present often covers both meanings:

• radim = I work or I am working

The time expression ovaj tjedan makes it clear that this is about a current, ongoing situation:

• Ovaj tjedan radim prekovremeno = This week I’m working overtime

So Croatian relies more on context than English does here.

Prekovremeno means overtime or extra hours beyond normal working time.

In this sentence it is used adverbially with radim:

• raditi prekovremeno = to work overtime

A related noun phrase is:

• prekovremeni rad = overtime work
• prekovremeni sati = overtime hours

So radim prekovremeno is a very natural way to say I’m working overtime.

Because the speaker is saying:

• I am working overtime
• because we have too much work

This is very natural. It suggests that the speaker is part of a team, company, department, household, or group. So the reason is shared by a group, even though only the speaker is being talked about in the first clause.

If the speaker wanted to say I have too much work, they would say:

• jer imam previše posla

But jer imamo previše posla means because we have too much work.

Because after quantity expressions like previše meaning too much / too many, Croatian normally uses the genitive.

So:

• previše posla = too much work

Here posla is the genitive singular of posao.

This is very common in Croatian:

• puno vremena = a lot of time
• malo novca = little money
• previše ljudi = too many people

So previše posla is exactly what you would expect grammatically.

The base form is posao, meaning job, work, or task/workload, depending on context.

Its forms can look a little irregular to English learners:

• nominative singular: posao
• genitive singular: posla

So in this sentence:

• imamo previše posla = we have too much work

Here posao is being used in the broad sense of work to do, not necessarily a job in the employment sense.

Because jer introduces a subordinate clause, and in standard Croatian that clause is normally separated by a comma.

So:

• Ovaj tjedan radim prekovremeno, jer imamo previše posla.

This is standard punctuation.

In English, we often do not need a comma before because, but Croatian punctuation rules are different here.

Yes. A very common alternative is zato što.

So you could also say:

• Ovaj tjedan radim prekovremeno, jer imamo previše posla.
• Ovaj tjedan radim prekovremeno zato što imamo previše posla.

Both mean This week I’m working overtime because we have too much work.

Jer is shorter and very common in everyday speech. Zato što is also very common and may sound a little more explicit.

No, Croatian word order is fairly flexible, although some orders sound more natural depending on emphasis.

The given sentence is very natural:

• Ovaj tjedan radim prekovremeno, jer imamo previše posla.

But you could also say:

• Jer imamo previše posla, ovaj tjedan radim prekovremeno.

This version puts more emphasis on the reason first: Because we have too much work, this week I’m working overtime.

Croatian often moves elements around to highlight what is most important, while the case endings and verb forms keep the meaning clear.

Yes. In standard Croatian, tjedan is the normal word for week.

Learners may also encounter sedmica, but that is less standard in Croatian and is more associated with other regional varieties, especially Serbian and Bosnian usage.

So for standard Croatian, ovaj tjedan is exactly the form you should learn.