B737 Captain reported returning to the gate after the push back crew noticed an engine oil service access door was open. Contract maintenance was called and reported both engine oil service caps were left uninstalled. The oil caps were installed correctly and the flight crew continued in normal operations.

Narrative

Prior to boarding; I was notified by the Operations Agent that our aircraft had arrived from ZZZZ a few hours prior; had been serviced for oil by contract maintenance and had been towed to the gate from a remote location. During push back and engine start; one of our wing walkers noticed that the oil fill access panel on the #2 engine had come open. We returned to the gate and I went downstairs to secure the panel. When I looked inside the panel I saw that the oil fill cap on the engine was off and was dangling by the retention chain. I contacted dispatch and maintenance to arrange for contract maintenance to secure the oil fill cap. The Contract Maintenance Technician entered the cockpit and informed me he had secured the oil cap on the #2 Engine and secured the access panel. He also stated that he checked the #1 Engine; just in case and found the nacelle access panel closed and latched but when he opened the panel to check the oil filler cap he found it off and dangling by the retention chain. BOTH OIL FILL CAPS HAD BEEN LEFT OFF OF THE ENGINES AFTER SERVICE. The Maintenance Technician replaced the oil fill cap and closed and latched the access panel. We continued with our scheduled flight.