> It's not that it was needed to 'fill a gap' in any language. Before 1839, English speakers had 'yes,' 'good,' 'fine,' 'excellent,' 'satisfactory' and 'all right.' What OK provided that the others did not was neutrality, a way to affirm or to express agreement without having to offer an opinion.
That, as is then explained - response without judgement, is a really big and useful gap to fill.