This gist shows how using the pre-request script in Postman, a new Oauth-2 token can be obtained using the a refresh token

postman-oauth2-refresh-token.js

/**
** Inspired by: https://stackoverflow.com/a/52436395/1756953 
** Modified from: https://gist.github.com/harryi3t/dd5c61451206047db70710ff6174c3c1
**
** Postman as of Nov 2018 does not support auto-refreshing of Oauth-2 tokens.
** This is an exmaple on how in one can refresh their Oauth-2 tokens just using the pre-request scripts.
** Pre-requisites: You need to have a refresh token. You can use the Postman app to get one.
**
** Step 1: Create a Postman environment and set the following variables:
**    tokenUrl
**    clientId
**    clientSecret
**    refreshToken
** If you already have a valid accessToken, add:
**    accessToken
**    accessTokenExpiresAt - Unixtimestamp access token will expire at (so it is not regenerated until it expires)
**
** Step 2: Edit Collection or Request, under "Authorization" set type to OAuth 2.0 and set "Access Token" to: {{accessToken}}
**
** Step 3: Copy the code below into the "Pre-request Scripts"
**
**/

// Only run if access token is expired
if (pm.environment.get('accessTokenExpiresAt') > new Date().getTime()/1000) return;

// Set all these variables in an environment or at collection level
let tokenUrl = pm.environment.get('tokenUrl'),
    clientId = pm.environment.get('clientId'),
    clientSecret = pm.environment.get('clientSecret'),
    refreshToken = pm.environment.get('refreshToken'),
    requestOptions = {
      method: 'POST',
      url: tokenUrl,
      body: {
        mode: 'formdata',
        formdata: [
            {
                key: 'grant_type',
                value: 'refresh_token'
            },
            {
                key: 'client_id',
                value: clientId
            },
            {
                key: 'client_secret',
                value: clientSecret
            },
            {
                key: 'refresh_token',
                value: refreshToken
            }
        ]
      }
    };

console.log({ requestOptions });

pm.sendRequest(requestOptions, (err, response) => {
  let jsonResponse = response.json(),
      newAccessToken = jsonResponse.access_token,
      newRefreshToken = jsonResponse.refresh_token,
      expiresAt = Math.floor(new Date().getTime()/1000) + jsonResponse.expires_in;
      
  console.log({ err, jsonResponse, newAccessToken });
  
  // To persist the tokens
  pm.environment.set('accessToken', newAccessToken);
  pm.environment.set('refreshToken', newRefreshToken);
  pm.environment.set('accessTokenExpiresAt', expiresAt - 60)
});