Note that out of these 22 Olympiads 2 of them (Bangla & ICT) don't have any international segment and some Olympiads are still accepting registration. I have attached those Facebook event links as well as the list of the available websites. You can find the syllabus and everything from there yourself. Hope it helps. Also, I will be entertaining the queries about these in the comments as long as it is within my abilities to answer.
Olympiad Name : Inter School Bangla Olympiad
Segments : 5
Eligibility : Must be a student of a school ( Up to grade 10)
Timeline : Around December
Website : www.banglaolympiad.org
Facebook page : https://www.facebook.com/banglaolympiad
Olympiad Name : Bangladesh Junior Science Olympiad (BDJSO)
Categories : 4
Eligibility : Grade (3-12)
Timeline : Around July
Website : www.bdjso.org
Facebook page : https://www.facebook.com/bdjso
Olympiad Name : Bangladesh Astro Olympiad (BdAO)
Categories : 2
Eligibility : Grade (8-12)
Timeline : Around June
Event Link : https://fb.me/e/2q8brbO0D
Facebook page : https://www.facebook.com/astronomybangla
Olympiad Name : Bangladesh Biology Olympiad (BdB0)
Categories : 4
Eligibility : Grade (3-12)
Timeline : Around January
Website : www.bdbo.org
Facebook page : https://www.facebook.com/bdbo.org/
Olympiad Name : Bangladesh Physics Olympiad (BdPhO)
Categories : 3
Eligibility : Grade (6-12)
Timeline : Around January
Website : www.bdpgo.org
Facebook page : https://www.facebook.com/BdPhO/
Olympiad Name : Bangladesh Robot Olympiad (BdRO)
Categories : 2
Segments : 5
Eligibility : Age (7-18)
Timeline : Around July
Website : www.bdro.org
Facebook page : https://www.facebook.com/BdRobotOlympiad
Olympiad Name : Bangladesh Math Olympiad ( BdMO)
Categories : 4
Eligibility : Grade (3-12)
Timeline : Around January
Website : www.matholympiad.org.bd
Facebook page : https://www.facebook.com/BdMOC/
Olympiad Name : IQ Olympiad
Categories : 12
Eligibility : Grade (1-12)
Timeline : Around July
Event link : https://fb.me/e/2naEJVwzy
Facebook page : https://www.facebook.com/iqolympiadbd/
Olympiad Name : Bangladesh Economics Olympiad (BdEO)
Categories : None
Eligibility : Grade (8-12)
Timeline : Around March
Facebook page : https://www.facebook.com/BdEO.Official/
Olympiad Name : Blockchain Olympiad (BdBCO)
Categories : 4
Eligibility : Grade (8-Graduates)
Timeline : Around May
Website : www.bcolbd.org
Facebook page : https://www.facebook.com/BCOLBD/
Olympiad Name : Bangladesh Informatics Olympiad (BdIO)
Categories : None
Eligibility : Grade (8-12)
Timeline : Around December
Website : www.olympiad.org.bd
Facebook page : https://www.facebook.com/BdOIOfficial/
Olympiad Name : ICT Olympiad (ICTOBd)
Categories : TBA
Eligibility : TBA
Starting time : TBA
Website : TBA
Facebook page : https://www.facebook.com/ictolympiadbd
Olympiad Name : National AI Olympiad
Categories : 2
Eligibility : Grade (3-12)
Timeline : Around June
Facebook page : https://www.facebook.com/iqolympiadbd
Olympiad Name : Coding Olympiad
Categories : 6
Eligibility : From age 4 to 23
Timeline : Around September
Event link : https://fb.me/e/1NzuC41rH
Facebook page : https://www.facebook.com/CodingOlympiad
Olympiad Name : English Olympiad Bangladesh (EOB)
Categories : 6
Eligibility : Grade (1-12)
Timeline : Ongoing
Website : www.englisholympiad.net
Facebook page : facebook.com/EnglishOlympiadBangladesh
Olympiad Name : Bangladesh General Knowledge Olympiad
Categories : 5
Eligibility : From Class 1 to Masters Level
Timeline : Around October
Event link : https://fb.me/e/3BoLdQhPA
Facebook page : https://www.facebook.com/bdgko.ypn
Olympiad Name : National Earth Olympiad (NEO)
Categories : None
Eligibility : Grade (8-12)
Timeline : Around March
Website : https://www.byei.org/neo2022
Facebook page : https://www.facebook.com/EarthOlympiad
Olympiad Name : National Newspaper Olympiad (NNO)
Categories : 5
Eligibility : Grade (3-Masters)
Timeline : Around December
Website : www.nnobd.org
Facebook page : https://www.facebook.com/NNOBD.Official
Olympiad Name : National Climate Science Olympiad (BDCSO)
Categories : None
Eligibility : Age (14-17)
Timeline : Around July
Facebook page : https://www.facebook.com/BdCSOlympiad/
Olympiad Name : Bangladesh Olympiad on Astronomy and
Astrophysics
Categories : None
Eligibility : Grade (9-12)
Timeline : Around March
Facebook page : https://www.facebook.com/bdaa.olympiad/
Olympiad Name : International Nutrition Olympiad
Categories : 2
Eligibility : Age (10-24)
Timeline : Around October
Website : www.nutritionolympiad.net
Facebook page : https://www.facebook.com/InternationalNutritionOlympiad2021
Olympiad Name : Bangladesh Philosophy Olympiad
Categories : None
Eligibility : Grade (6-12)
Timeline : Around February
Website : http://www.ipo-bangladesh.org/
Facebook page : https://www.facebook.com/IPObangladesh/