You can find several tables in OpenX database that could be very large in comparation to other tables. The database could keep growing constantly.
These are the four largest tables in OpenX database:
You can delete records from the ox_data_summary_ad_hourly and ox_data_summary_zone_impression_history but in that case all statistics data of the campaings will be lost.
Tables ox_data_raw_ad_impression and ox_data_intermediate_ad are temporary tables that keep records for the five days. After five days records will be transfered to ox_data_summary_ad_hourly table, so before you delete records from this table check whether data were transfered.