Is there a way to get disc space remaining using an SQL query on an Oracle database?
You should look here at Managed Tablespace Alerts, notably this bit where you can monitor by two criteria with warning and critical thresholds.

    By percent full

For both warning and critical thresholds, when space used becomes greater than or equal to a percent of total space, an alert is issued.

    By free space remaining (in kilobytes (KB))

For both warning and critical thresholds, when remaining space falls below an amount in KB, an alert is issued. Free-space-remaining thresholds are more useful for very large tablespaces.