Please refer to the last release of RM0433 which contains update and correction.
The contradiction in the RM0433 Rev6 mentioned by waclawek.jan was already corrected in the Section: "Single ADC operating modes support when oversampling" to remove the mention that the offset correction is not supported in oversampling mode.
For those who still looking for the offset implementation in oversampling mode I paste here a line from a program that seems to work:
if (HAL_OK == HAL_ADC_Stop_DMA(&hadc2))
{
LL_ADC_SetOffset(hadc2.Instance, LL_ADC_OFFSET_1, LL_ADC_CHANNEL_3, offset << 10); // 10 At the end means that you use 10 bit shifting oversampling
}
This has something to do with where exactly this offset would be substracted - before oversampling or after oversampling. An RM states that clearly somewhere. Don know exact place, but it is there.