With respect to the appearance of the savior, I ought to point out that in accordance with the perspective of Shiite, 12th Imam of Shiite (Imam Mahdi (a.j.)) is going to appear to save the world from the oppression. So my question is that what is the viewpoint of Sunni regarding this issue? In truth, do they believe in this matter as well? If so, who is him (according to Sunni)?
It's the same for both Sunni & Shia. It's the same religion. On one side you have Wahhabies (in their different forms), they love Sahaba but they don't appreciate A'al Albayt, they even deny (or hide the proof of) their high status.
On the other hand, there are Shia who love Ahlul-bayt but they're misinformed about some of the Sahaba and hate them accordingly. Right in the middle, there are Sufis, they love all Ahlul-bayt and all the Sahaba. All sects are awaiting Imam Mahdi's appearance.
Al-Mahdi in Sunni Ahadith Collections
Many of the Sunan Collections have a whole chapter about a deliverer:
In Sunan Abi Dawod (for example1, 2, 3 which is Naming him al-Mahdi, 4 which is describing him) it is quoted that their will be a deliverer from Ahl Al-Bayt (Peace be upon them) with different grades of authenticity.
Just as an Indication for the 1st one: By abu Bakr and 'Omar, abu Dawod doesn't refer to the two Sahabis (May Allah be pleased with them) but to different Chains one narrated by Abu Bakr ibn 'Ayyash and the other 'Omar ibn Abayd!
Ahadith on 'Isa Ibn Maryam's appearanceT
There are also ahadith quoting 'Isa ibn Maryam (Peace be upon him) (See for example here in Jami' at-Tirmidhi) as a Person who would play a role in this.
This Hadith which one could find in Both sahihs is precising that 'Isa will descend while al-Mahdi will be among us (1,2,3)
Sunni View about al-Mahdi
Sunni believe that at the End of time the Mahdi will appear as you may read in the above listed Ahadith/Hadiths where he is named, described and also a hint of where he will come (or appear) from is given. And He will rule the earth in accordance with Islamic Shari’ah. But they don't worship by waiting for him to appear. This is an accepted and well known matter among Sunni scholars.
For example al-Shawkaani said:
"The ahadith that we have come across that speak of the awaited Mahdi reach the level of tawaatur, including fifty saheeh, hasan and da’eef munjabar ahaadeeth. There are mutawaatir without a doubt; indeed they are more mutawaatir than others, according to all the standards of usool. There are also many reports from the Sahabah that speak about the Mahdi and that reach the level of marfoo’, so there is no room for individual opinions in such an issue."
But on the other hand one should be aware that some liars have fabricated adhadith about the Mahdi, and other even claimed to be he (One of them might be al-Mahdi ibn Tumart).
And Allah knows best!