Document and Entity Information	Total
Prospectus:
Document Type	485BPOS
Document Period End Date	Apr. 30, 2019
Entity Registrant Name	BNY Mellon Opportunistic Municipal Securities Fund
Entity Central Index Key	0000797923
Entity Inv Company Type	N-1A
Amendment Flag	false
Document Creation Date	Aug. 28, 2020
Document Effective Date	Sep. 01, 2020
Prospectus Date	Sep. 01, 2020
BNY Mellon Opportunistic Municipal Securities Fund \| Class I
Prospectus:
Trading Symbol	DMBVX
BNY Mellon Opportunistic Municipal Securities Fund \| Class Y
Prospectus:
Trading Symbol	DMBYX
BNY Mellon Opportunistic Municipal Securities Fund \| Class Z
Prospectus:
Trading Symbol	DMBZX
BNY Mellon Opportunistic Municipal Securities Fund \| Class C
Prospectus:
Trading Symbol	DMBCX
BNY Mellon Opportunistic Municipal Securities Fund \| Class A
Prospectus:
Trading Symbol	PTEBX

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Regulation 12B
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-Section 12
-Subsection b-2

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