I have failed ordinary level Maths subject in 2017. Can I register for advanced level in 2018 before I get the ordinary level results in 2018?

Yes you can.
But you need to request it from the examination department.
Call and get more information from the hotline which is 24 hr service.

Address : Department of Examinations, Pelawatta, Battaramulla, Sri Lanka

Hotline : 1911

Tel No. : +94 11 2786200, +94 11 2784201, +94 11 2785202, +94 11 2784204, +94 11 2786205, +94 11 2784206

Fax : +94 11 2785220 (General), +94 11 2784422 (School Exams), +94 11 2784232(Agency Exams)

It is possible to do local A/L as a private candidate without local O/L. But there is a criteria as a prerequisite to have O/L subjects relevant to the A/L subjects if you are going sit through a school. And also it may be an issue when applying for local Universities.

You can do A/L without doing national O/L but the problem is the knowledge - exam format and the choosing subjects related to O/L.
You will face many difficulties unless you prepare yourself well for the exam .
Think twice.

I am doing my o/l (2019) for the first time this year. Unfortunately if I failed my o/l exams can I do a/l ?

I have done my O/L exam in English Medium in 2020. But I have not maths subject in that year. And i can't done 2nd shy also. When I ready to apply for the 3rd time I missed that chance. Because the valid time is closed. But I am doing A/L's. Can I sit for the coming O/L exam? Can I get emergency admission

I have 4 s passes and 1 a pass and 1 c pass
Can I do al in Sri Lanka ?

I have 2 B passes and 1 C pass and 6 s passes in O/L. Can do art stream in A/L?