Bid normally if possible, but don't bid NT without a stopper in the opponents' suit. If the overcall means you can no longer make your normal bid, Pass.

If opponents double partner's 1NT: with under 9 HCP and a 5+ card suit, run to it (longest first, then strongest, then lower level). Otherwise Pass.

If opponents double partner's suit opening for takeout: with a fit, bid one level higher than you normally would. Without a fit, Pass.