$ opam switch list-available 'ocaml-b*'
# Listing available compilers from repositories: default
# Name              # Version     # Synopsis
ocaml-base-compiler 3.07          Official 3.07 release
ocaml-base-compiler 3.07+1        Official 3.07+1 release
[..]
ocaml-base-compiler 5.2.0~beta2   Second beta release of OCaml 5.2.0
ocaml-base-compiler 5.2.0~rc1     First release candidate of OCaml 5.2.0
ocaml-base-compiler 5.2.0         Official release 5.2.0

$ opam switch list-available 'ocaml-base-compiler'
# Listing available compilers from repositories: default
# Name              # Version     # Synopsis
ocaml-base-compiler 3.07          Official 3.07 release
ocaml-base-compiler 3.07+1        Official 3.07+1 release
[..]
ocaml-base-compiler 5.2.0~beta2   Second beta release of OCaml 5.2.0
ocaml-base-compiler 5.2.0~rc1     First release candidate of OCaml 5.2.0
ocaml-base-compiler 5.2.0         Official release 5.2.0

$ opam switch list-available 'ocaml-base-compiler.4.14.0'
# Listing available compilers from repositories: default
# No matches found

$ opam list 'ocaml-base-compiler.4.14.0'
# Packages matching: name-match(ocaml-base-compiler) & version-match(4.14.0) & (installed | available)
# Package                  # Installed # Synopsis
ocaml-base-compiler.4.14.0 4.14.2      Official release 4.14.0