Every single Olympia that I have seen in the UK has a 5mm diameter spindle.  This is to take the standard German 'DIN 2103' spools that fits almost every German-made typewriter since long before the Second World War.  The Germans standardised their typewriter spools between different makes very early on.  As an aside, this spool was adopted by an increasing number of non-German manufacturers throughout the 1950's and 1960's and was in danger of becoming a world standard until fashions changed and mechanical typewriters fell from favour.  Olivetti, of course, always went their own way.  From what I have read on the internet, I have deduced that Olympia (and Truimph-Adler I suspect) fitted special 4mm spindles for the American market, so that their machines could take the widely-available Underwood-style spool.  Clearly Hermes changed over to using this style of spool after the war to capture more of the US market, and both Brother and Silver-Reed (who were copying the Hermes Baby/Rocket to an extent) also adopted this spool for the same reason.  So it nearly became a second world standard.  It says a lot that the three styles of typewriter spool that are still in production in China are DIN 2103, Underwood, and the Olivetti spool.

Re: Reddit  Yes that is exactly the problem.  The guy has spindles set up for the DIN 2103 spool and has been supplied with Underwood-style spools.  A previous owner has crammed an Underwood spool onto the left spindle and broken it.  The suggestion of an Olivetti spool wouldn't help.  That one has a 6mm hole, so would be too slack !

The DIN spools used to be marked, then the Germans changed the DIN number to DIN 32755 - so some spools are marked like that.  The Chinese spools, and many others, have no markings at all.  All have a 5mm certre hole and three drive holes.  The Underwood-style spools have a 4mm centre hole and four drive holes.

Not neccessarily.  This spools fits Hermes, Brother and Silver-Reed.  I'm sure that these must have been imported to Germany at some stage.