When should I put comma before OR?

Traditionally, conjunctions are set off with commas, but the trend has been to omit the commas.

Thanks to Infra but I think you will need it because without it the ambiguity stops a speedreader in his tracks

That is after rewriting to satisfy the local Dumb Bunny (me) who thinks there still might be a grammatical error of some sort

So Blue shouldn't that have been 'OR, is the'

Can you give an example?

I'm thinking of:

Either speak up or sit down.

No comma needed.

What about independent causes separated with these conjuctions?

2/3 of them are actually Russian-speaking descendants, who either repatriated after the USSR collapsed, or are just looking for temporary employment.

And by the way, is it correct to use past simple tense in one clause and present continuous in the second clause? Seems like, it's a grammatical mistake, too, because clauses in parallel constructions have to be the same type.