“Am i considered in-state if i can prove that i have lived in virginia 12 months prior to attending the first day of college?”
“can i be considered in-state for states i have been born in and/or have lived in with my parents for a reasonably long period of time.”
No. Residency is basically about where you pay taxes. You have to have be a current taxpayer there.
Virginia is very used to out of state students trying to game the residency system. Read the rest of the link that bomeer gave you, and accept that those are the rules. As you are under 24, the residency of your legal guardian(s) is the criteria that counts. If your aunt and uncle are your legal guardians (see here for how they could be come so: http://www.ehow.com/how_10038856_obtain-guardianship-virginia.html) then you can be considered a resident. Happymomof1 is right that it will be important for somebody to have legal guardianship no matter what: even getting medical care will be a problem as long as you are under 18.
“what are my chances of being admitted given the fact that i have lived in the US for 12 years and bangladesh for 4 years.”
Admitted to what? to secondary school? 100%. To a college? depends completely on the college and your qualifications. To 12th grade? anything is possible, but as other posters have pointed out, it would not be likely to be the best plan.
O-levels are given at the end of the equivalent of 10th grade, so you would be in essence skipping a year - and note that bearpanther is mistaken that O-levels are AP level (at least, not as they are now).
Also, having moved teenagers from another country to the US in the last 5 years, there are few other points that might be helpful:
=> schools: as you are taking O levels, you must be in a private school, and therefore are probably studying in English (with maybe one class in Bengali / other languages?), which may keep you from having to take the proficiency test that momof1 mentions. However, whether you apply to a private secondary school or go to the local public school you will be given placement tests in math and any languages you have studied.
=> bearpanther is right that math is a particularly difficult subject for transferring- our kids have transferred in and out of various school systems (Europe/SA/NA) and math was always the hardest adjustment (closely followed by foreign languages). It’s not that one place or system is harder or easier than another, it’s that the elements are taught in different orders, so one week you are ahead (because you have done that concept) and the next you are suddenly lost (because the material includes a unit you haven’t done yet). Very frustrating, but you can get through it.
=> If you are coming to Northern Virginia the public school system may have some familiarity with O levels, but even there you will have to do a lot of the matching up of qualifications for course placement. If you are going anywhere else in Virginia, it will be even more work (unless there is a Bangladeshi community locally).
=> When you look at what courses you sign up for get some advice on what the colleges you want to apply to are likely to want to see on your transcript.