1. Previous Employer - As per the OINP Application Guide 'OINP requires that you have a minimum of one year of continuous and full-time employment (1,560 hours or more) or an equal amount in continuous part-time paid work experience in a National Occupation Classification (NOC) level 0, A, or B occupation in the past five years from the date of the PT Notification of Interest from Ontario.' So you meet this requirement. Try to get the reference letter in the format that I mentioned in the above comment in addition to employment contract/offer letter, salary slips/pay stubs, relieving letter, etc.
2. Current Employer - I understand the underlying risk that you have mentioned. There are two ways you can handle the situation.
a) Option 1 - Try explaining to your current employer that it is not that you are going to leave them immediately, it is a process that would take 6-8 months and you can give a commitment to them that you will not quit. There is nothing wrong in you trying for a better future and you are no way going to a competitor, you are planning to migrate to another country. If this works, this would make your life simple, you can get the letter from them (and if required, to make them feel that you are genuine, you can get them to add a clause in the letter saying that, 'This letter is issued for the purpose of immigration'). This approach has worked for me, but it all depends on the way you handle the situation and the counter person.
b) Option 2 - If option 1 does not work, then try to get a letter from a colleague (preferably) a senior person who can vouch for your roles and responsibilities. You can also get it notarized and also attach his/her visiting card and copy of his employee id to the letter.
Whatever works for you, submit all other documents (employment contract/offer letter, salary slips/pay stubs, relieving letter, etc) with the reference letter with a LOE (Letter of Explanation).
