Flight instructor reported while entering the ATC pattern descending from an ATC assigned altitude resulting in ATC issuing an immediate turn to avoid a collision with another aircraft. ATC vectored the aircraft back to the pattern where they landed safely at the airport.

Narrative

On Day 0 I was returning to SNA from ZZZ approaching the airport from the Huntington Beach pier. As usual; the Tower Controller on 126.8 instructed me to cross over the control tower at or above 1300 feet and make left traffic for Runway 20L. As I approached about a half mile from the tower; the controller said; 'cross over the Tower at 1500 feet; and contact Tower on 119.9'. The latter is the Tower frequency for Runway 20 left. I repeated the instruction and tuned the new frequency; which was very congested with no opportunity to announce my presence. After crossing the field; as usual we reduced power and began descending and turning left for the downwind for Runway 20L; where pattern altitude is 860 feet. The Tower Controller on 119.9 instructed me to make an immediate right hand turn toward the east for traffic. As I complied; I noted another aircraft off my right that I had to turn steeply to pass narrowly behind. The Tower Controller asked me why I descended. He asked me if I was given the crossing altitude of 1500. I replied yes; we were instructed to cross over the tower at 1500 to make left traffic for 20 left. He replied that I was given an altitude restriction and should have stayed at that altitude. I complied and continued east bound until the controller eventually vectored me back toward the airport to enter the left traffic pattern and land safely.As a pilot at John Wayne; I'm incredibly grateful to our controllers; who work extremely hard to keep us safe. These guys and gals are really top pros. It's challenging at any airport when there are two Tower frequencies and aircraft are switching from one to the other after receiving an instruction from one controller that will be carried out in the domain of another. At best; the altitude and pattern entry instructions I received from ATC were very ambiguous. It was not at all clear to me why the first controller amended my altitude to cross over the Tower at 1500 instead of the 1300 that he had first instructed. After the fact; I understand it was because there was traffic in the downwind at pattern altitude; and he wanted me to pass over that traffic; but neither controller rescinded the clearance I had received to turn left and enter the traffic pattern; so I followed the instruction at the amended altitude.In this case; clear instructions would have avoided confusion and a potential accident; particularly because the controllers needed me to deviate from the usual pattern entry procedure.