By documentation [1] function indexof returns the zero-based character position of the first occurrence of the second parameter value in the first parameter value.
Function substring doens't have explicitly specified indexing, but based on Example 60 [2] is function substring zero-based too.

indexof
Reference service which supports OData V4:
ULR: http://services.odata.org/V4/Northwind/Northwind.svc/Customers/?$filter=indexof(CompanyName,'lfreds') eq 1
Result: Customer with CompanyName equals "Alfreds Futterkiste"

URL: http://localhost:8080/odata4/olingo_basic/Source/SimpleDataTable/?$filter=indexof(stringval,'alue') eq 1
Result: No entities
Expected result: Entity with intkey 1,2,3,4,8,9,10,11,12

substring
Reference service which supports OData V4:
URL: http://services.odata.org/V4/Northwind/Northwind.svc/Customers/?$filter=indexof(CompanyName,'lfreds') eq 1
Result: Customer with CompanyName equals "Alfreds Futterkiste"

ULR: http://localhost:8080/odata4/olingo_basic/Source/SimpleDataTable/?$filter=substring(stringval, 1) eq 'alue_asdf'
Result: No entities
Expected result: Entity with intkey 10

[1] https://docs.oasis-open.org/odata/odata/v4.0/errata02/os/complete/part2-url-conventions/odata-v4.0-errata02-os-part2-url-conventions-complete.html#_Toc406398121
[2] https://docs.oasis-open.org/odata/odata/v4.0/errata02/os/complete/part2-url-conventions/odata-v4.0-errata02-os-part2-url-conventions-complete.html#_Toc406398122