Members of the OGA and being an amateur golfer would make anyone eligible to play in any OGA competition. There are some additional restriction such as handicap or age for some competitions. There are some tournaments that are by invitation and there are some that require players to qualify for. The OGA Tour on the other hand is open to anyone regardless of membership or amateur status.
On each individual tournament page it will note what the eligibility requirement is. Below is an example of one might look like:
Eligibility: Entries are open to amateur golfers with Handicap Indexes of 10.0 or less. The final Championship field will include exempt players as well as those that advance from the additional 18-hole stroke play qualifying stages. All contestants must be members in good standing of an OGA Member Club and be at least 25 years of age as of April 29, 2023