B737-800
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 160; Flight Crew Total 1896; Flight Crew Type 1896
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Aircraft Equipment Problem Dissipated; Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft
During powerup for takeoff we noticed a strong odor in the cockpit. Because we hadn't noticed it earlier; it seemed that it could be coming from another aircraft or externally. After takeoff it was apparent it was coming from our aircraft; but dissipated during climbout so we decided to continue. During the descent below 23;000 feet; the odor was apparent again but not as strong. The engines were at idle. We were close to ZZZ at this point so we reviewed the fumes in the cockpit checklist and discussed donning the O2 mask and advising ATC should the situation become worse. Below 17;000 feet we started the APU and used the bleed.  It helped for a few minutes but the odor returned. The flight attendants also smelled the odor in the cabin. The Gate Agent immediately mentioned the odor as she opened the door. The engine oil was serviced in ZZZZ and the engines were washed.
B737 Captain reported a strong odor on takeoff which initially dissipated during climbout and returned during descent at destination.