OD operation involves 5 components and the wiring that connects them...

1) Fuse 11 (200) or 12 (700/900) supplies voltage to items #2, #3, #4, and #5

2) The Shifter button/switch turns the OD relay off and on

3) The OD relay controls the Upshift (not "OD") Arrow light and Solenoid (separate wires)

4) The Upshift Arrow light ON means the relay and OD are OFF

5) The OD solenoid allows OD engagement when ON, (the "default" Relay state at Key On)

If the Shifter switch turns the Arrow light (#4) on and off, then items 1, 2, and 3 are OK, and the problem is most likely the undercar wire to the Solenoid (#5) or the wire connector terminal at the solenoid.

Less likely would be be Solenoid itself.
Also possible (but unlikely) would be the relay controlling the Arrow light (#4) correctly, but failing to activate the Solenoid.
An internal transmission problem is also possible.

Consider this recent reply to a "check the wiring" suggestion:
"Replaced corroded connector and length of wire with bad insulation. Working perfectly now."
He was having "intermitent" OD failures, and was sure it "couldn't be wiring" because that would be "all or nothing".

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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.