The Treatise on the Passover dates from around 245. Its central insight is that the passover is not a figure or type of the passion of Christ, but a figure of Christ himself, of Christ's passing over to the Father. The Dialogue with Heraclides probably comes from between the years 244 and 249. It seems to be the record of a synod-like meeting of bishops, in the presence of lay people, called to discuss matters of belief and worship. Both pieces seem to come from the last decade of Origen's activity, when he was at the height of his powers.
Treatise on the Passover and Dialogue with Heraclides
Ancient Christian Writers
Mind, Body & Spirit