Despite what the general public thinks now, the US Navy did have an idea that an attack was possible and that they should be prepared in Hawaii for one. A series of missteps in communication led to the fleet at Pearl Harbor not being pushed out to sea, and a fighter screen being put up around the base prior to the Japanese attack.

In fact, mere hours before the Japanese planes took off from their stationed aircraft carriers, U.S. intelligence decoded a Japanese cable instructing their ambassador in Washington to break off diplomatic negotiations by 1 p.m. December 7, and to destroy his code machine. Chief of Staff of the Army General George Marshall read this correctly as a sure sign of imminent attack and sent a message forward to all of his commanders. Yet the message didn’t reach the officials in Hawaii until an hour after the strike—several hours after it should have arrived—because a clerk failed to mark the message “top priority,” and it was sent via commercial telegraph rather than through naval wireless communications.

There was an official investigation launched just after the attack in 1942 led by the American Supreme Court (things worked differently back then.) It laid the blame very heavily on the top Navy and Army commanders in Hawaii: Admiral Husband E. Kimmel and General Walter C. Short. Why? War with Japan had been imminent for months (tensions across the world were rising) and the US Navy stronghold in Hawaii was one of the obstacles that Japan would have to deal with. Both men had received several messages in the days and weeks prior to 12/7/41 that an attack was imminent yet they left Pearl Harbor basically defenseless to an aerial attack. Both men were ‘convicted’ of Dereliction of Duty and forced into an early retirement.

Source: At Dawn We Slept by Gordon Prange et. al.

The other response gets lots of the context right, especially the fact that while the specifics of war breaking out were something of a shock, its actual fact very much was not. Especially for senior military leadership in the Pacific.

Halsey had taken ENTERPRISE and a task force of escorts to reinforce Wake Island with Marine Wildcats a week before and was operating under the assumption that he would fire on any Japanese forces which approached him as they could have no good intent. He also declined the option given to him by Kimmel to take a trio of battleships that in the event of war would normally be his to command, reasoning them too slow and fuel hungry to fight his task force to his liking.

As for your question about actual defenses around Oahu it comes down to shortages of material compared to the mission, and divisions in cross and internal service responsibilities. The Pacific Fleet may have been based at Pearl but was not the primary organization charged with the defense of the base and islands themselves. To do so would force the fleet to dedicate resources and wear themselves out when the entire purpose of having a major base there was to allow the fleet to be prepared to sortie for a better position in the Central Pacific. The basic security task then fell to the 14th Naval District for the most part, who was in charge of shore installations in Hawaii. However there were never enough ships to do every job called for so often a few destroyers would have to be detailed from the fleet's strength to bolster picket duties. A similar strain existed for those valuable eyes fo the fleet in the Patrol Squadrons of long ranged aircraft. The AOR stretched from Guam and Wake Island to Samoa, to Hawaii and up to Alaska. And even then for each individual base of course a smart foe could approach from almost every point on the compass and wear and tear on man and machine had to be balanced against density of search flights.

But we can also note the obvious, there were pickets out to guard the fleet at Pearl Harbor, and they even did their job. The USS WARD, a destroyer of WW1 vintage assigned to the 14th Naval District was on duty off the entrance to Pearl when she responded to reports of a possible sub by a Coast Guard manned vessel. She then fired on what was eventually confirmed to be one of the gaggle of IJN midget subs attempting to infiltrate the harbor, in the early 2000's the wreck was discovered with a shell hole from WARD's deck gun in her conning tower.

But a handful of ships could only patrol the immediate area of Pearl's entrance, which of course is on the Southern Coast of the island while the Kido Butai approached from VAST emptiness of the North Pacific. And area patrolled at various times and in various density by both Navy-Marine Corps and Army Air Force long ranged aircraft which were never in sufficient numbers, and could only patrol out so far and with airborne radar still only in the very early experimental phase for the US, by launching just before dawn the Kido Butai could get the jump on most scouting flights from Oahu.