Similar to SQLServer where I can do the following
create index TimeSeriesPeriodSs1 on TimeSeriesPeriod (validationStatus, completionStatus)
where completionStatus= N'Complete'
and validationStatus= N'Pending'
You can create a function-based index in Oracle that leverages the fact that NULL values aren't stored in b-tree indexes. Something like
CREATE INDEX TimeSeriesPeriodSs1
ON TimeSeriesPeriod(
(CASE WHEN completionStatus = 'Complete' AND validationStatus = 'Pending'
THEN validationStatus
ELSE NULL
END),
(CASE WHEN completionStatus = 'Complete' AND validationStatus = 'Pending'
THEN completionStatus
ELSE NULL
END)
);