BOD, is by definition the amount of oxygen demand over a certain time period (BOD5 is incubated for 5 days) so I can't see that you'll be able to get the exact BOD instantly.
However w-water are advertising BOD analysis within 3-4 hours with their CheckLight Early BOD kit: https://www.w-water.com/uploaded_files/documents/checklight_bod_datasheet_u2727.pdf
I understand the BOD BART system can analyse BOD in 20 hours: https://www.dbi.ca/books/PDFs/BOD-Paper.pdf
And headspace BOD can get values similar to the BOD5 in 2-3 days: https://www.engr.psu.edu/ce/enve/logan/publications/2000-Logan-etal-HBOD.PDF
I have no experience of this, but COD can be measured more rapidly and may be used to estimate an equivalent BOD: https://www.aquadiagnostic.com/downloads/brochures/L100.pdf
Alternatively you could perhaps measure the causative agents of the demand more rapidly and in the field - for example TOC: https://www.geinstruments.com/products-and-services/toc-analyzers-and-sensors/5310-c-portable https://www.hach.com/anatel-a-1000-xp-portable-toc-analyzer/product?id=7640480104
If you just need to know the instantaneous dissolved oxygen concentration (DO), I have had good experience with Hach's LDO probes, otherwise their BOD track apparatus could be more appropriate: https://www.hach.com/bodguide-measurement
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