Optimizing a query that gets the opening balance
I created a minimal, complete, and verifiable example for your query.

CREATE TABLE oss_collection_history
(
    org int
    , Balance int
    , DateFrom date
);

INSERT INTO oss_collection_history VALUES (1, 1, '2019-01-01');
INSERT INTO oss_collection_history VALUES (1, 2, '2019-01-02');
INSERT INTO oss_collection_history VALUES (2, 1, '2019-01-03');
INSERT INTO oss_collection_history VALUES (2, 2, '2019-01-04');

CREATE INDEX oss_collection_history_Charges_IDX2 
ON oss_collection_history (org, DateFrom, Balance);


This query returns a single row-per-org, with the associated opening balance, using the index:

SELECT och1.org
    , och1.Balance
    , och1.DateFrom
FROM oss_collection_history och1
INNER JOIN (
    SELECT MIN(och2.DateFrom) AS MinDateFrom
        , och2.org
    FROM oss_collection_history och2 
    GROUP BY och2.org
) t ON och1.org = t.org AND och1.DateFrom = t.MinDateFrom;


Results:

╔══════╦══════════╦════════════╗
║ org  ║ Balance  ║  DateFrom  ║
╠══════╬══════════╬════════════╣
║   1  ║       1  ║ 2019-01-01 ║
║   2  ║       1  ║ 2019-01-03 ║
╚══════╩══════════╩════════════╝

This fiddle for this is here.