Hi Asivad,
Im a new member too and it seems you are once of the most helpful and well informed members of the board.
1) Whats your opinion if i have the invitation to both express entry stage 2 to submit my documents (invitation received today) and the Ontario provincial nominee program (express of interest received last week)? as i understand, i can only apply for 1 of them at a time an while the first one allows me 60 days to submit my docs, the second one only 45 days.
An ITA is what everyone on the forum wants second only to PPR so Congratulation on landing one! OOPNP is a provincial program to help you land a nomination which will give you 600 points so that you are guaranteed an ITA in the next draw. Since you already have an ITA, you don't need those 600 points and, by extension, the nomination or the invite you have received. Focus on submitting your application within the next 60 days.
2) is there any advantage of the Ontario nominee program compared to direct express entry?
Explained this above. No advantage if you already have an ITA.
3) Someone told me if i apply for Ontario provincial nominee program and if they are interested by my documents, they may even offer me a job letter?
You have been misinformed. The province doesn't help you get a job. The invitation might lead to a nomination which will lead to an ITA which might lead to a PR visa which will help you to live and work in Canada but you'll have to do the legwork required to get a job.
4) Does a provincial nominee acceptance increase my chances of getting my documents approved in Stage 2 ? (I got rejected last time because i didnt provide Australian Police Clearance Certificate along with rest of my documents due to a misunderstanding)
Not at all. Provinces also have the same rules of eligibility as CIC and, in some cases, are stricter than CIC with some of them (not OOOPN though) requiring applicants to have either a job offer in the province or by rewarding applicants who can demonstrate past/present connections with that province. You will still have to submit your application to CIC after getting the nomination and that's when the PCCs will be required to be procured and submitted. As I said earlier, focus on the ITA and the application submission. The forum is extremely helpful and will help you with solutions to most, if not all, of your queries.
Thanks!